When the aircraft generator controller detects effective voltage value, calculation error is often caused by frequency fluctuation. Three kinds of effective value algorithms are introduced, which are constant frequency sampling algorithm, zero crossing algorithm and quasi-synchronous sampling algorithm. Simulation in MATLAB shows that among the three algorithms, the quasi-synchronous sampling algorithm leads to the minimum error. For the problem of the quasi-synchronous sampling algorithm response time is too long, the semi-periodic integral optimization method is adopted, and the number of iterations is appropriately reduced. The results of simulation and experiment show that when the frequency fluctuates between 380 Hz and 420 Hz, the error of the improved quasi-synchronous sampling algorithm is optimized from 2.63% to 0.14% compared with the traditional constant frequency sampling algorithm. The response time is reduced from 7.42 ms to 2.46 ms compared with the quasi-synchronous sampling algorithm without optimization.