A method and a device of starting applications installed on a mobile operating system (OS) is performed by a mobile device for the mobile OS to flag any application installed on the mobile OS as a windowed application and display the application under a multi-window mode when the application is started. When multiple applications are simultaneously started, the mobile OS sequentially determines if each application is a windowed application. If positive, the application is displayed under the multi-window mode, and the size and location of the displayed window frame with the application running thereon are adjustable, rendering higher operational efficiency and convenience of software applications installed on a mobile OS.