A method is developed to retrieve total cloud optical thickness (COT) from global solar radiation (GSR) detected by ground-based pyranometer, and approaches to input aerosol/molecular/gas parameters for COT retrievals are presented. On the basis of numerical simulations and comparative tests, main error factors of COT retrievals are analyzed, which include radiation data error, cloud inhomogeneity, uncertainties of aerosol optical parameters, and surface albedo. The retrieved COT error, caused by a --5% or 5% systematic error of the GSR measurement, is within ¿0.6 and ¿5.0 for COT ranges of 0--5.0 and 5--100, respectively. The AOT, the aerosol single scatter albedo (SSA), and the surface albedo are three significant parameters affecting COT retrieval accuracy. The mean SSA in the pyranometer spectral response range and the broadband surface albedo are suitably used in the retrievals. If uncertainties of AOT, SSA, and surface albedo are within ¿0.1, ¿0.05, and ¿0.05, respectively, the retrieval accuracy is accepted for most applications. Furthermore, COTs (τPyr) from pyranometer data at two meteorological observatories are compared with COTs (τISCCP) from ISCCP and COTs (τMODIS) from MODIS. The relative standard deviations between monthly mean τPyr and τMODIS, or τPyr and τISCCP, are all less than 45.4% for both sites. The agreement among the yearly mean τPyr,τMODIS, and τISCCP is satisfactory. The absolute (relative) deviations between the yearly mean τPyr and τMODIS are within ¿1.55 (8%) for both sites, and the deviations between the τPyr and τISCCP are within ¿1.94 (25%). The yearly mean τPyr also agrees considerably well with τISCCP in the broken cloud case.