把功能写在方法里，函数化，方法化

main函数

var grade = "<!--{$grade}-->";
        //var grade = 0;
        var lesson_id = "<!--{$lesson_id}-->";
        //var lesson_id = 0;
        if(grade!=""&&lesson_id!=""){
            getBookByGradeAndLesson(grade,lesson_id);
        }
        
        var book_id = "<!--{$aQuestions.book_id}-->";
        var chapter_id = "<!--{$aQuestions.chapter_id}-->";
        var section_id = "<!--{$aQuestions.section_id}-->";
        if(book_id!=""){
            getChapterByBookId(book_id);
            if(chapter_id!=""){
                $("#chapter_id").find("option[value="+chapter_id+"]").attr("selected",true);
                getSectionByChapterId(chapter_id);
                if(section_id!=""){
                    $("#section_id").find("option[value="+section_id+"]").attr("selected",true);
                }
            }
            $("#book_id").find("option[value="+book_id+"]").attr("selected",true);
        }
        
        //年级变动
        $("#grade").change(function(){
            var grade = $(this).val();
            var lesson_id = $("#lesson_id").val();
            if(grade!=""&&lesson_id!=""){
                getBookByGradeAndLesson(grade,lesson_id);
            }
            chapterRefresh();
            sectionRefresh();
        });
        
        //科目变动
        $("#lesson_id").change(function(){
            var lesson_id = $(this).val();
            var grade = $("#grade").val();
            if(grade!=""&&lesson_id!=""){
                getBookByGradeAndLesson(grade,lesson_id);
            }
            chapterRefresh();
            sectionRefresh();
        });
        
        //书变动
        $("#book_id").change(function(){
            var book_id = $(this).val();
            if(book_id!=""){
                getChapterByBookId(book_id);
            }
            //chapterRefresh();
            sectionRefresh();
        });
        //章变动
        $("#chapter_id").change(function(){
            var chapter_id = $(this).val();
            if(chapter_id!=""){
                getSectionByChapterId(chapter_id);
            }
        });

方法体

function getBookByGradeAndLesson(grade,lesson_id){
            var url = "/default/index/ajax/do/getBookByGradeAndLesson";
            var data = "grade="+grade+"&lesson_id="+lesson_id;
            $.ajax({
                type:"POST",
                async:false,
                url:url,
                data:data,
                success:function(response){
                    if(response){
                        $("#book_id").html(response);
                    }else{
                        alert("error");
                    }
                }
            });
        }
        function getChapterByBookId(book_id){
            var url = "/default/index/ajax/do/getChapterByBookId";
            var data = "book_id="+book_id;
            $.ajax({
                type:"POST",
                async:false,
                url:url,
                data:data,
                success:function(response){
                    if(response){
                        $("#chapter_id").html(response);
                    }else{
                        alert("error");
                    }
                }
            });
        }
        function getSectionByChapterId(chapter_id){
            var url = "/default/index/ajax/do/getSectionByChapterId";
            var data = "chapter_id="+chapter_id;
            $.ajax({
                type:"POST",
                async:false,
                url:url,
                data:data,
                success:function(response){
                    if(response){
                        $("#section_id").html(response);
                    }else{
                        alert("error");
                    }
                }
            });
        }
        function chapterRefresh(){
            var html = "<option vlaue=''>---请选择---</option>";
            $("#chapter_id").html(html);
        }
        function sectionRefresh(){
            var html = "<option vlaue=''>---请选择---</option>";
            $("#section_id").html(html);
        }

主函数调用方法体里的函数功能块，这样写，使得代码清晰，易修改和维护。

以后要多多这样写代码，不要把所有的代码都堆成一堆，乱起八糟的。