In order to allocate airport slots efficiently and fairly, the auction theory was used to design slot auction mechanism, including allocation rules and payment rules. Firstly, aming at the 2-2 slot trading, winner determination model was presented and solved. Secondly, the payments of airlines and traffic flow management department were calculated, following the vickrey payment rule. Thirdly, several new payment rules were proposed to meet the demand of budget balance. Finally, a example simulation was carried out. The results show that combinatorial auction can satisfy the airline's decision-making objectives and the new payment rules can meet balanced budget requirements.