Abstract: Through the energy attenuation method of combining two cuneiform prisms and a neutral attenuator, the polarization of COIL whose power density is 20 W/mm2 was measured. The corner cubes were used as the retroreflectors of stable cavity, thus the uniformity of laser power was improved. The laser beam irradiated the two prisms with 45° incident angle and were reflected by the cuneiform prisms. Then the beam passed normally through a neutral attenuator, and was received by a polarimeter at last. Experimental results indicate: At zero electromagnetic field the light of transverse flow of two modules of COIL without any polarizing elements is partially polarized if the resonator configuration is a foldaway stable cavity, moreover, the degree of polarization decreases gradually in the course of