In order to solve the problems of the linear response of the detector and the poor stability of the UV light source in the process of bidirectional reflectance distribution function bidirectional reflectance distribution function measurement of the far ultraviolet spectrum (140~240 nm) for solar diffuser, a relative method of measurement using the light source monitoring proportional compensation was proposed. According to the method, a system based on the structure of six-degree-free turntable was designed. The system can measure the bidirectional reflectance distribution function of the solar diffuser at any point and any position in the hemisphere space by controlling the diffuser with two-dimensional translation and rotation and the detector with two-dimensional rotation. The measurement experiment was carried out by using the proposed method, and the main factors that affect the measurement results of the system were analyzed. The total uncertainty of measurement is about 5.5%.