The optimization of capacity allocation of traditional microgrid has limited effect on improving economic efficiency. To solve this problem,a microgrid model based on the coupling of landfill gas power generation (LGPG) and power to gas (P2G) is propoed to improve economic efficiency. Considering the influence of carbon trading and the time value of capital on the optimal allocation of capacity, the full life cycle theory and net present value analysis method in economics are introduced. Then the mutation particle swarm optimization algorithm is used to obtain the optimal allocation of capacity in four scenarios for maximizing the benefit of the whole life cycle. The optimal operation scheme is obtained by Cplex under the optimal allocation of capacity of the traditional microgrid model and the proposed microgrid model. The simulation results show that the economic efficiency of proposed microgrid model based on LGPG-P2G is higher than that of tranditionl microgrid model under the background of carbon trading.