A system for automatically placing virtual advertisements in sports videos, which includes a shot detection module, background extraction module, a calibration module and an asset placement module. The shot detection module detects the target shot of a sports video via a first trained model. The background extraction module performs a background extraction to extract the background of the first frame of the target shot to obtain a first background mask. The calibration module performs a camera calibration to detect a first transformation relation, between the first frame and a sport field template, via a second trained model. The asset placement module transforms an advertisement asset according to the first transformation relation to obtain a first transformed asset, and execute an asset placement to place the first transformed asset onto the first frame according to the first background mask to obtain a first image frame with placed advertisement.