Because the optical components in the high energy laser system can be evaluated by measuring the transmittance, such as the detection distance, damage ability and energy loss degree of the system, we used the common optical-path composite displacement synchronous-scanning technique via point-by-point scanning to determine the transmittance rate of large-diameter components. The upper and lower mechanical scanning structures are controlled to achieve fast scanning of the full aperture by composite displacement scanning in the circumferential and radial directions, combined with the calibration of the optical path using a galvanometer. The experimental results show that the measurement accuracy of optical element transmittance measured by the proposed method is 0.2%, the measurement aperture reaches up to 600 mm, and the measurement speed exceeds 500 h^-1. These values satisfy the technical specification requirements and achieves high-precision measurements of transmittance of large-diameter laser components.