转自：http://blog.sciencenet.cn/blog-562235-1083517.html
编译Qt时，Qt路径极被hard-code到Qt库里面去了(见下文详细描述)。此时如果更改Qt路径(比如将已编译好的库挪动)，极有可能出现找不到plugin，甚至找不到Qt库的错误。遇到类似问题，可以通过qt.conf文件指定新的路径来解决。在qmake.exe所在目录创建一个qt.conf 文件(如果文件不存在)，通过Prefix指定新的Qt路径，例如：
[Paths]
Prefix = D:/Qt/Qt5.9.2
The qt.conf file overrides the hard-coded paths that are compiled into the Qt library. These paths are accessible using the QLibraryInfo class. Without qt.conf, the functions in QLibraryInfo return these hard-coded paths; otherwise they return the paths as specified in qt.conf.
Without qt.conf, the Qt libraries will use the hard-coded paths to look for plugins, translations, and so on. These paths may not exist on the target system, or they may not be accesssible. Because of this, you need qt.conf to make the Qt libraries look elsewhere.
See “Using qt.conf” in the Qt Documentation.