轉自：http://blog.sciencenet.cn/blog-562235-1083517.html
編譯Qt時，Qt路徑極被hard-code到Qt庫里面去了(見下文詳細描述)。此時如果更改Qt路徑(比如將已編譯好的庫挪動)，極有可能出現找不到plugin，甚至找不到Qt庫的錯誤。遇到類似問題，可以通過qt.conf文件指定新的路徑來解決。在qmake.exe所在目錄創建一個qt.conf 文件(如果文件不存在)，通過Prefix指定新的Qt路徑，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.