Space optical communication demands harsh power budgets. Since the actual channel characteristics of the atmosphere is very complex, the channel power budgets tends to introduce large deviation with the general link equation. By the comprehensive analysis of the factors of effecting characteristics of atmospheric channel, such as atmospheric attenuation, turbulence effects and refraction, etc. Combined with a large number of the derivation and calculation, a correction method of the link equation is put forward to get a link equation that positively responds to the channel characteristics. By this equation, budgets optical power for the actual ground optical communications with the link distance of about 6 km and the bit error better than 10-8. Experiments and analysis show that the link equation correction method is reasonable, the power budget is consistent with experimental results, and the near ground optical communication is equivalent demonstration to satellite-ground optical communication.