A method of determining relative spectral responsivity of photodetector by LED-based spectrum-tunable integrating sphere source is put forward, and the measuring principle and algorithm are exhaustively described. In the course of calculation, the radiant transmission integral equation was changed into summation formula, and the degree of approximation between integral value and summation value is related to the selected wavelength interval. The differences between integral value and summation value in different wavelength intervals of Si photodiode and CCD were simulated and analyzed. The simulated results demonstrated that the relative differences between signal integral value and signal summation value of Si photodiode and CCD were below 0.2% in 10 nm interval, so 10 nm interval was an ideal choice. In the end, the factors affecting measurement accuracy were discussed and the solution suggestions were given. This method is easy in structure, and it avoids the measurement transmission errors of some instruments, such as monochromator.