The invention provides an automobile crossbeam assembly hole visual on-line measurement system and a method thereof. The system comprises a main control platform, a servo motor, a servo motor encoder, a slide platform, a slide block, a horizontal slide rail, a fixed high and low frame, a camera and a measured crossbeam. A host is arranged on the main control platform. A motion control card and an image acquisition card are respectively inserted in PCI insertion slots of the host. The host is connected with the servo motor through the motion control card. The servo motor encoder is coaxially connected with the servo motor. The servo motor is fixed on the slide platform. The slide platform is matched and connected with the horizontal slide rail through the slide block. The horizontal slide rail is arranged on the high side of the fixed high and low frame. The host is connected with the camera through the image acquisition card. The camera is arranged on the slide platform through a Z type height adjustable bracket. The measured crossbeam is arranged on the low side of the fixed high and low frame. According to the invention, non-contact and full-automatic high speed on-line measurement is carried out on the hole diameter and the hole site size of an automobile crossbeam assembly hole; and the system and the method are especially suitable for the measurement of the crossbeam with nonuniform assembly hole distribution and without assembly holes in certain areas.