In the electricity market， the network structure has a great impact on the market performance. Therefore， the transmission network planning becomes more important. The heuristic method of adding line one by one is applied in this paper to make transmission network planning in the electricity market under pool model， in which the producers will bid strategically according to the forecasted demands. The budget limitation and new isolated buses are considered in the planning. The sensitivity index in the approach is defined not only by the point of construction cost， but also by the market efficiency. Compared with the traditional methods， an 18-bus system simulation proves the feasibility of the proposed approach， and gives a dynamic transmission network planning prediction in 8 years.