To develop a high repetition rate nanosecond narrow-pulse light source with with pulse width, repetition frequency and output power tunable to the single-photon level for the visible wavelength with high repetition frequency, a green frequency-doubling light source design based on a gain-modulated DFB laser and a PPLN crystal is proposed. The DFB laser is driven by a homemade nanosecond high-speed driver circuit in a pulsed mode to generate a fixed-wavelength pump source; using a fiber optic semiconductor attenuator to adjust the attenuation amount and modulate the intensity of pump light; using PPLN crystal as a nonlinear crystal material, a quasi phase matched green frequency doubling laser output was obtained through its tunable properties. The experimental results show that the pump power and center wavelength of the gain modulated DFB laser are stable, with a pulse width of 1.02~10.25 ns and an operating frequency of 1 Hz~10 MHz that can be continuously and independently adjusted. When the power of pump light decays to -44.61 dB, the average photon number of the green frequency doubling light output pulse is about 384N, and the conversion efficiency of the frequency doubling light to light is 9.3%.