Radiometric calibration in-flight is one of critical techniques for information quantification of optical remote sensing. With the development of quantitative application of high spatial resolution satellite optical sensors, required precision on absolute radiometric calibration is asked higher and higher. A calibration method based on gray-scale tarps is presented. The measured ratio of diffusion to global irradiance substitutes the aerosol scatter computed by radiance transfer code. Besides high reflectance tarps are deployed to improve the accuracy of on-orbit radiometric calibration. Preliminary results show that in-flight absolute radiometric calibration using gray-scale tarps for high spatial resolution satellite sensors less depends on model assumption and could achieve high precision calibration at full dynamic range. The uncertainty is less than 4% and this approach can also satisfy the application demand to the complex environment.