A blind spot detection alert system. The system comprises two image capturing modules, a processing module, a first display module and a second display module. The two image capturing modules capture a right image and a left image respectively. The first display module is disposed on the right of the second display module. The processing module receives the right image and the left image to analyze a car moving direction. When the processing module determines the car moving direction is a right direction, the first display module displays the right image. When the processing module determines the car moving direction is a left direction, the second display module displays the left image. The processing module calculates a distance to a coming car, a moving direction of the coming car and a speed of the coming car according to the image to get an alert message.