Abstract: The interaction between space plasma and spacecraft results into the onset of spacecraft surface charging and the resultant electrostatic discharging events. The computation of spacecraft surface charging is commonly accomplished using the secondary electron yield of spacecraft surface irradiated by mono-energetic electrons. To depict the charging environment more precisely and obtain more reliable computation results, focusing the spacecraft charging problem under the worst charging condition and taking into accounts the double-Maxwellian plasma distribution, the threshold equation controlling the onset charging is derived based on the averaged secondary electron yield. This equation is useful to analyze spacecraft charging under the condition of election irradiation with a continuous energy spectrum. Besides, the adoption of double-Maxwellian plasma distribution could better model the space plasma condition in the case of magnetospheric substorm. By theoretical analysis, the ambient plasma is divided into two typical situations according to the charging characteristics of spacecraft surface charging. Through simulation computation, the trend of spacecraft surface charging versus plasma parameters fluctuations is obtained for these two typical situations. Results show that higher electron temperature corresponds to more severe charging with higher negative potential; meanwhile, the density ratio of the two electron components in double-Maxwellian plasma distribution plays an important role in spacecraft surface charging. The obtained conclusions could provide useful reference for quantitative analysis of spacecraft surface severe charging events.