As a market-based environmental pollution control system,emission trading system,with dual attributes of both public law and private law,consists of initial emission allocation,emission trading contract,and government supervision. Emission trading contract,at the heart of the system,reveals its nature of civil law. The primary market of emission trading is the initial allocation of environmental capacity resources by government,reflecting the wills of the state,while the secondary market,as a real market mechanism,is the redistribution of environmental capacity resources among discharging units,reflecting the autonomy of will. At present,people focus more on point source transaction contracts between units,but in future,there is a substantial tendency for trading contracts between point sources and non-point sources. The emission trading contract is different from traditional civil contract in subject,object,forming and effect. The behavior of the contract subject may involve several types of legal responsibilities,which are characterized by concurrence,and the concurrence of responsibilities is not a choice,but to apply at the same time,to ensure a complete relief for the interests damaged.