A spectroradiometer incidence system was designed to realize the coupling of direct irradiance. In order to reduce energy loss between light-collecting barrel and fiber, and solve the problem of low energy level in direct coupling, the characteristic dimensions of the barrel were restrained, the lightcone parameters were calculated according to the geometrical optics theory. The coupling efficiency in different wavelengths and change of efficiency under different tracking angle errors were tested by solar simulator. The theory analyses indicate that axis’s lateral offset of lightcone and fiber is the main factor that caused coupling efficiency loss. Experimental results indicate that the coupling efficiency of the whole system is 66.24%, the efficiency is 12.8% when the sun tracker miss the sun completely, which reflects the level of circumsolar radiation and scattered radiation. The system can satisfy the requirements for the observation of direct solar irradiation.