Abstract: The influence of the source in transmittance meter on the farfield spot of probe beam was analyzed in theory. Combined with the principle of luminescence for LED source,the surface characteristics of LED and the relationship between surface characteristics and measurement of transmittance meter were studied . We proved that the surface characteristics of LED could influence the optical path collimation and the stability of measurement by simulation experiment. The results of the study show that the surface characteristics of LED in transmittance meter can influence the energy distribution of the farfield spot of probe beam and make the center position of probe beam offset 25 mm with the baseline of 50 m, the maximum variation of receiving power is 20%, and hence it can influence the optical path collimation of transmitting terminal and the measurement of the transmittance meter. The energy distribution of the farfield spot can become more uniform and the maximum variation of receiving power is 1% by increasing diffusion chip and aperture.