Based on self-developed measuring equipment, a method for measuring the reflection spectrum of crop canopy using active light source was discussed. As well as systematically introduced the measuring principle, five functional modules of the system were introduced. They were high-frequency modulated light source module, first-level amplifier circuit module of the photoelectric cell response, high pass filter module, second-level amplifier circuit module and signal extraction module. Theoretical analysis and derivation on the design of the two key parts, the first-level amplifier circuit module and the signal extraction module, were provided. Furthermore, the self-developed experimental equipment was used to measure the reflection spectrum in wheat field. The experimental results showed that the impact from passive light source on the measuring results was almost completely filtered, while the response caused by active light source was detected efficiently. Experimental results were stable and had a small fluctuation ratio of less than 4.97% during experiments.