Disclosed is a device with precise channel spacing and multiple wavelength light source with high flatness and a method, belonging to the technical field of the optical communication, comprising an adjustable seed light source, a radio frequency signal source, a single arm mach tsend modulator, a double parallel mach tsend modulator, a power spliter, three electrical amplifiers and two electrical phase shifters. The method comprises: cascading the single arm mach tsend modulator and the double parallel mach tsend modulator, respectively driven by a low speed radio frequency signal, through selecting the bias points of the two modulators respectively as the highest point and the lowest point of the transmission curve, and simply control the amplitude and the phase of the radio frequency signal of the modulator, obtaining the wavelength light source including nine wavelengthes with the frequency interval equal to the frequency of the radio frequency signal. The wavelength light source of the invention has good spectral purity and phase coherence, high flatness, without expensive high power high frequency amplifier, thereby greatly reducing the arrangement cost.