Uncertainty exists in the planning program, so effective evaluation on and prevention from risk help to improve the guidance function of planning. This paper firstly classifies the risky factors into three major categories including exploration stage, development stage and the constrainted; as well as six subcategories including resources fulfillment, exploration pace, production building pace, production operation, constraints of economic profit, constraints of policy, middle-stream and downstream. Meanwhile the meaning of each type of risk is analyzed. On basis of Monte Carlo method, a quantitative risk assessment model for oil and gas development planning is established, whose object is the randomly distributed key index of development planning. The final case evaluation shows, this model can not only draw the future development of the evaluated region, but also simulate the scope and probability of development scale at each stage; therefore, it reflects intuitively the scale of risk in the evaluated region to provide effective supports for decision-making.