The proportion of renewable energy such as wind power in power system is increasing gradually, which puts forward a new peak shaving challenge for power system. The deficiency in peaking capacity will seriously harm the reliable and stable operation of power system, and limit the consumption of renewable energy to a large extent at the same time. In view of the good peaking advantage of natural gas units, this paper argues that the gas turbine is suitable to be worked as one of the main peaking power source in the grid. The estimation model of peaking capacity of gas unit is established, considering the impacts of random renewable energy generation, like wind power, and load on peak-shaving demand; conditional value-at-risk (CVaR) theory is introduced to do better risk control of the uncertainty of power system peaking capacity; finally, the rational allocation of system peaking capacity is realized, considering the peaking ability of all kinds of peaking power sources. An actual situation of the power system in a certain area is analyzed in China, and the result shows that the model can effectively calculate the peaking capacity of natural gas unit under a certain confidence level and margin of peak-shaving, and provide reference for power source planning. This work is supported by the China Sustainable Energy Program of Energy Foundation (No. G-1406-21484).