Setting the project basic reserve fund is one of the important approaches to control the project risks.Usually,a fixed rate is used to calculate the fund.Because the fund is used to deal with the risk that may result in additional capital expenditure,the fixed rate method may not reflect the reality situation accurately.In this paper,Monte Carlo simulation method is implemented in which the contingency is calculated by simulating decomposition investment to measure the distribution of unforeseen expenses.By the specific characteristics of decomposition,the quantitative risk analysis basis is formed,while the contingency become more rational,scientific and effective.