uniapp默认横屏启动
pages.json文件配置

"globalStyle": {
    "navigationBarTextStyle": "black",
    "navigationBarTitleText": "uni-app",
    "navigationBarBackgroundColor": "#F8F8F8",
    "backgroundColor": "#F8F8F8",
    // 横屏启动
    "pageOrientation": "landscape"
}

app单个页面横屏
plus.screen.lockOrientation( 'landscape-primary');
或者
// plus.screen.lockOrientation( 'landscape-secondary');