CSDN上找到答案
到网上下载DroidSansFallback.ttf
然后
int fontId = QFontDatabase::addApplicationFont(":/font/DroidSansFallback.ttf");
if(fontId != -1){
QStringList androidFont = QFontDatabase::applicationFontFamilies(fontId);
if(androidFont.size() != 0){
QFont font(androidFont.at(0));
app.setFont(font);
}
}