The invention provides an optical network unit reflected light modulator for a TWDM-PON (Time and Wavelength Division Multiplexed Passive Optical Network) system. The reflected light modulator comprises a U-type waveguide and a coupled micro ring resonator, wherein the U-type waveguide comprises a first branch and a second branch; and the coupled micro ring resonator is coupled between the first branch and the second branch and comprises two mutually-coupled ring resonators. The coupled micro ring resonator receives uplink seed indicator optical signals comprising multiple uplink seed wavelengths via the first branch. The coupled micro ring resonator is configured to filter out target uplink seed wavelengths from the uplink seed indicator optical signals, modulate uplink data to the target uplink seed wavelengths so as to form uplink optical signals, reflect the uplink optical signals to an optical line terminal via the first branch, and output the remaining of the uplink seed indicator optical signals to the air via the second branch.