The invention discloses a laser transmission welding system and method, and relates to the field of laser welding. The system comprises a laser emitter, a vacuum clamping system and a control system.The vacuum clamping system comprises a coordinate platform, a clamping device and a workpiece system. According to the laser transmission welding system and method, lasers are adopted as energy, and the sealing performance of an inner cavity of the vacuum clamping device is guaranteed through a sealing ring, a sealing gasket and a sealant; a sealing cavity of a box body is vacuumized to generate negative pressure in the clamping device, a piston rod is driven through the inner and outer pressure difference of the box body to apply clamping force to the workpiece system, and accuracy of the clamping force is controlled through a sensor-computer-relay; and laser transmission welding between similar or dissimilar thermoplastic plastic and metal is achieved, and welding quality is improved.