With the self-developed measurement equipment of canopy reflection spectrum, the spectral stability of two kinds of narrowband-LEDs, which were used as active light sources and had peak wavelengths of 730nm and 810nm respectively, were studied. The experimental results showed that the junction temperature of the LEDs was significantly increased when the LEDs were pulse-lighted in high frequency based on the request of the measurement. The increased junction temperature resulted in linear red-shift of the light source spectra and linear decrease of luminous intensity of the light source, which seriously affected the stability and reliability of the measuring results. A new method of driving LEDs, named intermittent pulse-driving LED, was proposed. When LEDs were driven in this method, the rise of the junction temperature could be effectively controlled below 2℃, and the red-shift of peak wavelength of light source spectra was less than 1nm. Finally using the self-developed measurement equipment of canopy reflection spectrum, in which this method was embedded, the experiments were implemented both in laboratory and wheat field to measure the reflectance response of the crop canopy. The fluctuation ratio of measured results was less than 5%. The LED-driving method proposed could be applied in developing measuring equipments for detecting reflection spectrum of crop canopy. 