The optimal design of transmission system expansion planning is an important part of the overall planning task of electric power system under competitive electricity market environments. Taking the cost-benefit analysis theory in economics as a basis, this paper analyzes the economy and reliability of the power transmission network at the beginning. Then, a mathematical model cost/benefit is proposed, and it is calculated by genetic algorithms which is introduced in section IV. Finally, it is tested and analyzed by Gaver-6-node system. Furthermore, the results indicate the feasibility and effectiveness of the method proposed in the paper.