The invention discloses a screen projection method and system and an electronic device. The screen projection method is used for displaying multimedia data played on terminal device on playing equipment except the terminal device. The method comprises: acquiring multimedia data, processing the multimedia data into screen projection data, sending the screen projection data to a playing device to bedisplayed, the multimedia data comprising streaming media data, and the screen projection data comprising frame data with a target coding format; in other words, processing the multimedia data into the screen projection data including the frame data in the target coding format during screen projection, sending the screen projection data to the playing device, the playing device can directly achieve screen projection display of the multimedia data according to the screen projection data, and the user experience can be improved.