Vicarious calibration in large and uniform sites by the reflectance-based method is currently a common on-orbit calibration method. The limited number and low reflectivity of uniform sites in China have restricted calibration frequency and accuracy. The gray-scale target method has achieved high-precision radiometric calibration in a uniform site, but how to break through the single environmental background of a large uniform site to achieve high-frequency calibration is still a tough problem. On the basis of the classic irradiance-based method, this paper proposes an improved irradiance-based method suitable for gray-scale targets, analyzes a method of on-orbit radiometric calibration in complex environments, and experimentally verifies the method on an optical satellite. The results show that the proposed improved irradiance-based method, with a maximum difference of less than 3.5% from the reflectance-based method, can achieve on-orbit radiometric calibration in a complex background. It provides a technical approach for high-frequency and high-precision on-orbit radiometric calibration of optical satellites in China.