At first, to aim at the unbalance bidding skill of tenders under the bill of quantities bidding mode, this paper puts forward the bidding mode of bill of quantities based on composite sealed-bid auction by study on basis of sealed-bid auction bidding theory. Secondly, the authors make use of game theory to analyze the bidders' optimal offer strategy which is unbalance bidding under the bidding model of bill of quantities. Thirdly, it is proven that the bidding mode of bill of quantities can effectively reduce the adverse effect which is bring by the tenders' unbalanced bidding, and to reduce the investment. At last, the authors validate the results of model by using the example.