In order to adapt to the demand analysis of power grid frequency modulation capacity under the rapid development of new energy and serve the construction of power frequency modulation market, a new power frequency modulation capacity demand assessment method considering the fluctuation of new energy is proposed. The purpose of reserving power frequency modulation capacity is to smooth the power generation deviation during the real-time operation of the power grid and keep the power grid frequency stable. At present, the deviation of power generation and consumption is mainly caused by the fluctuation of power load and the fluctuation of new energy output. Based on the analysis of the volatility of new energy and load, this paper proposes a method of power frequency modulation capacity demand assessment based on the volatility assessment. In this method, new energy and load fluctuation are regarded as independent variables, and the demand of power frequency modulation capacity is determined with the goal of maximizing the fluctuation of power generation and consumption. Finally, an example is given to verify the effectiveness of the proposed method based on the actual grid data of a province. The calculation results show that the evaluation results obtained by this method increase by 12.5% compared with the traditional method, which is more in line with the power grid operation requirements under the large-scale development of new energy.