In the bill-of-quantity model, as there are some differences between bidding engineering quantity and completed engineering quantity, the unbalanced quotation adopted by bidding companies may lead to the probable risk to construction companies. Based on the thorough analysis on the bidding strategies of bidding companies, the optimal unbalanced bidding model was built to acquire the optimum price quotation. Then, with Monte Carlo simulation method, cost risk evaluation model of power transmission and transformation projects was built to meature the risk of construction companies. The appropriate measures were used according to the risk scale, which could minimize the happening of excess budgetary estimate. The research results have guiding significance to the engineering cost management, and will standardize the bidding rules for the health, reasonable and sustainable development of the construction of power transmission and transformation projects.