Abstract: In the process of project implementation, cost of labor, materials expenses, construction machinery fees may have large alteration. So in the bidding process, there is very big project risk based on the valuation of pricing by bill of quantities. The determination of the risk cost was analyzed and discussed and a quantitative model for the evaluation of expenses risk was built in this article based on the pricing of bill of quantities in building construction, combined with the actual case analysis. The results show that the comprehensive evaluation method for project risk cost is feasible.