To effectively suppress the influence of time-varying loads and insufficient model accuracy on the control performance of vtol UAV power systems, an ISTSM-ADRC strategy is proposed. By leveraging the characteristic that ADRC is not completely dependent on the accurate mathematical model of the system, a speed-loop expansion state observer（ESO） is designed to online estimate and compensate for the comprehensive system disturbances. The sliding mode surface function with strong robustness is adopted to replace the nonlinear error feedback link in active disturbance rejection control(ADRC), thus enhancing the robustness of the speed-loop controller. A novel super-twisting sliding mode （STSM） reaching law is developed to achieve adaptive gain, reduce chattering and strengthen robustness against disturbances. Step response tests, load mutation tests and parameter perturbation experiments are conducted. The results indicate that compared with STSM-ADRC, the steady-state error of the proposed strategy is reduced by 50% and the steady-state fluctuation range after parameter perturbation is decreased by 46%. These findings verify that ISTSM-ADRC exhibits better dynamic performance and stronger robustness than STSM-ADRC and PI control strategies.