The invention provides a down transmitter applied to a DWDM-PON system. The down transmitter applied to the DWDM-PON system comprises an optical comb frequency generating unit, a band-pass filter, an optical circulator, a first array waveguide grating and multiple semiconductor reflection type optical amplifiers. The first end of the optical circulator is connected with the optical comb frequency generating unit through the band-pass filter. The second end of the optical circulator is connected to the semiconductor reflection type optical amplifiers through the first array waveguide grating. The optical comb frequency generating unit is used for generating comb type light sources with an adjustable central wavelength and an adjustable wavelength interval. The band-pass filter is used for filtering the comb type light sources to obtain the multiple wavelength channels. The semiconductor reflection type optical amplifiers are used for locking the wavelength channels and modulating down data. The first array waveguide grating is used for carrying out wavelength division multiplexing on the modulated wavelength channels and then inputting the modulated wavelength channels into the optical circulator. The invention further provides a related system of the down transmitter applied to the DWDM-PON system. The down transmitter applied to the DWDM-PON system has the advantages that the optical comb frequency generating unit serves as a light source for injection, the line width is small, and the output power is large.