Aiming at the difficulty of parameter tuning of PID controller in pump-turbine unit adjusting system, a PID control method of pump-turbine unit based on modified beetle antennae search algorithm is proposed. First, the mathematical model of pump-turbine unit adjusting system with time lag under small fluctuation is constructed. Second, an improved beetle antennae search algorithm combining adaptive fractional calculus and adaptive update step size is proposed. Then, the improved beetle antennae search algorithm, beetle antennae search algorithm and particle swarm optimization algorithm are selected for designing the PID controller of adjusting system under no-load and load conditions respectively. Finally, the PID controller optimized by each algorithm is used for simulating the conditions of no-load frequency disturbance and load fluctuation of adjusting system. The results indicate that the proposed algorithm can adjust the most suitable PID controller parameters, and the control system achieves better control effect with different time lags. This study provides a new idea for the design of PID controller of pump-turbine unit adjusting system.