Abstract: A modified automatic cloud-screening method is developed using 1-min measurement of surface downwelling shortwave global and diffuse irradiance. Clear skies are detected firstly by peak frequency density of ratio (PFDR) method. Then the detected clear-sky irradiance measurements are used to fit the expected clear-sky irradiance as the function of the cosine of the solar zenith angle by the least squares fitting method. Afterwards, the PFDR method is applied to the ratio of observed total shortwave irradiance to the predicted values for the cloud-free sky condition with the fitting expression. The clear sky detection method is verified using Total Sky Imager data and observer reports. The result shows that the average accuracy is 90.9% for the solar zenith angle less than 75°. For the modified cloud-screening method, the uncertainties due to both diurnal changes in such variables as column water vapor，aerosol loading，systematic pyranometer offsets，and solar zenith variables are diminished. Cloud occurring frequency and cloud radiative forcing (CRF) on downward shortwave irradiance at ground are calculated based on the clear sky detection results for Xianghe and Taihu. The cloud occurring frequency and the CRF show a distinct seasonal variation with maximum in summer and minimum in winter. The annual average of CRF is -39.5 W·m-2 in Xianghe and -66.2 W·m-2 in Taihu. The seasonal average of CRF in Xianghe is -25.9 W·m-2 in spring, -70.9 W·m-2 in summer, -51.1 W·m-2 in autumn, and -10.8 W·m-2 in winter. And the seasonal average of CRF in Taihu is -84.6 W·m-2，-89.1 W·m-2，-50.2 W·m-2，and -44.1 W·m-2, respectively, from spring to winter.