Suppression of stray light is a crucial part of space-based telescope (SBT) design. The suppression of earth-atmosphere radiation (EAR) is complicated. In order to analyze the EAR of SBT in its ambient environment, a computational model of EAR irradiance on sunlight baffle, which is one of the key components of SBT, is constructed by taking a typical SBT into consideration and using the irradiation transmission theory. Based on the as-constructed model, distribution character of EAR on baffle under various operating conditions is simulated. And radiant flux of primary and secondary EAR at light aperture of telescope is also calculated. The computation results indicate that sunlight baffle is the major stray light source for SBT working on low earth orbit (LEO), and the secondary EAR from sunlight baffle affects much more than the primary EAR, which directly passes into telescope at light aperture. Therefore, serious consideration on the secondary EAR from sunlight baffle should be taken into the design of stray-light suppression.