An anti-collision warning system for a vehicle door includes a sensing module, a detection module, and a warning module. The sensing module is configured to sense whether a passenger has an intention to open a vehicle door and output a sensing result. The control module is coupled to the sensing module, the detection module and warning module, and the control module is configured to control the warning module to issue an alert based on information on traffic conditions from the detection module in response to the sensing result being YES. The anti-collision warning system can issue an alert to the passenger in a timely and effective manner when it senses that the passenger has the intention to open the vehicle door. Therefore, the passenger would be alerted and determine whether it is an appropriate time to open the vehicle door, thus preventing the vehicle door from colliding with an object.