With the diversification of load types and the increase of the proportion of new energy installed capacity, the power fluctuation between source and load increases the unknown random disturbance, which makes the frequency response of power system more complex and unpredictable. Therefore, a unit commitment method considering random frequency safety under the Random-step-dual-disturbance method is proposed. The frequency safety constraint represented by the lower limit of frequency confidence interval is covered. Based on nonlinear least square method with weight, the multi-machine frequency response model is nonlinear fitted, and the equivalent single-machine frequency response model is obtained. Based on the stochastic linear system theory, a system state space model including frequency was constructed for the equivalent single-machine frequency response model to determine the initial value of the state variables. The influence of the initial value of the state variables on the random frequency response under the superposition of high-power step disturbance and random disturbance is revealed, and the frequency confidence interval of the most serious frequency safety index is deduced. The IEEE 24 test system and Actual test system verifies the effectiveness of the proposed method and quantifies the effects of different weights, random disturbance and initial states, and the proportion of new energy sources.