Based on the traditional generation expansion planning for bulk power system, the issue on the generation expansion planning model of Microgrid is discussed in this paper considering lower carbon economy, and the carbon trading under Clean Development Mechanism (CDM) in "Kyoto Protocol" is taken into account. An improved genetic algorithm - matrix real-coded genetic algorithm is applied, in which the encoded method and the operator have been modified, so not only the global convergence but also the optimization speed are both updated. The simulation results show that the proposed model and improved algorithm are effective.