This paper proposes a risk driven project buffer allocation method which will allocate project buffer according to risks, schedule constrains and resource constrains between tasks. The result of simulation based experiments shows that compared with Critical Chain, the proposed method can remarkably reduce the frequency of plan change and minimize the negative impact to project duration. The method and the tool can help project managers to decide appropriate project buffer length and make better use of the limited buffer. The stability and trustworthiness of project plan schedule can be enhanced.