The effect of downwelling atmospheric radiation(R_ld) on grass land surface temperature(T_s) is analyzed based on the three-year observations of upwelling surface long-wave radiation(R_lu) and R_ld in 4～50 μm band on clear days at SKYNET Hefei site. Results indicate that the effect of R_ld on mean daily T_s are stronger in summer than in fall and the weakest in winter. The mean daily difference between apparent surface. temperature T_* and T_s ranges from 0.96 K to 1.41 K through all year, with value larger than 1.3 K in July and August provided that land surface emissivity ε equals to 0.98. The effect of R_ld on diurnal variation of T_s is weakest at noon, greatest at night with the diurnal distribution of T_*－T_s showing“V”shape for each season and the effect is most remarkable in winter. The larger the ratio R_ld/R_lu, the greater the effect of R_ld on T_s and the quantitative statistical relationship between the ratio R_ld/R_lu, surface emissivity ε and T_*－T_s are obtained under different seasonal conditions.