Ratioing radiometer (RR) is a key instrument which can track the on-orbit degradation in bidirectional reflectance distribution function (BRDF) of on-board solar diffuser (SD), and its response to sun angle of incidence (K factor) is an important parameter which need to be measured pre-launch. A brief overview of the principle of RR is provided in this paper, K factor of each band of RR is acquired after several round of tests, which take halogen lamp in the lab instead of sunlight outdoor as light source . Uncertainty of K factors at 470, 650, 825 nm are acquired on the base of the data and error analysis. Result shows that the uncertainty of the K factor at 470 nm is 0.48% and within 0.16% at 650 nm and 825 nm, which meet the requirement of K factor uncertainty, providing valid basis for estimation of on-orbit calibration uncertainty and correction of the calibration coefficient.