The conventional magnitude modulation method of laser diode(LD) has defects, such as unstable modulated magnitude and nonlinear distortion, therefore a new LD magnitude modulation method based on the real-time power feedback was proposed in this paper. Photodiode(PD) was employed to monitor the output power of LD. And the operating current of LD was adjusted automatically according to the monitored output power of LD, by which the output power of LD varied with modulation signal. Finally, a prototype was built and tested. In the range of 20 ℃ to 40 ℃, The -3 dB bandwidth of prototype was up to 20 MHz， the stability of modulated magnitude was less than 4% and maximum nonlinear error was only 0.1%. The results show that the stability of LD output power is improved; the nonlinear distortion of modulated wave is also reduced, with linear working range of semiconductor lasers expanded.