Abstract: In this paper,the yields of secondary electrons(true secon-dary emission and backscattering)for the incident electrons at differentenergy levels are determined by using weighted average,and the thresholdtemperature T* and critical energy E* of spacecraft materials in high-voltage charging are calculated with these results.The characteristics of cri-tical charging are in good agreement with the observed data.It is expoun-ded that the complicated properties of spacecraft charging are truly qua-litatively explained by using the two concepts of T* and E*.