The invention provides an optical device and an optical signal processing method. A first light splitting part splits a light source to obtain a first optical signal, a second optical signal and a third optical signal; a first MZ modulator drives the first optical signal according to the first clock signal so as to modulate the first optical signal and output a fourth optical signal; a second MZ modulator drives a second optical signal according to the second clock signal to modulate the second optical signal, and outputs a fifth optical signal; a first optical coupler couples the fourth optical signal and the fifth optical signal and outputs a sixth optical signal and a seventh optical signal; a power regulator and a phase shifter perform power regulation and phase shift on the third optical signal to output an eighth optical signal; a second light splitting part divides the eighth optical signal into a ninth optical signal and a tenth optical signal; a second optical coupler combinesthe sixth optical signal and the ninth optical signal to offset the residual signal of the sixth optical signal; and the third optical coupler combines the seventh optical signal and the tenth optical signal to offset the residual signal of the seventh optical signal. Therefore, the quality of output signals is improved.