Four-level pulse amplitude modulation (PAM4) plays a more and more important role in high-speed serial link systems and receives more and more attention. The traditional traveling wave Mach-Zehnder modulator (TWMZM) has a spectral ratio of 1∶1. In this paper, a traveling wave Mach-Zehnder modulator with asymmetric light intensity (AP-TWMZM) is proposed. Its optical power spectral ratio at the input side is 4∶1. The PAM4 optical signal is generated by the upper and lower arms modulation to realize the optical domain PAM4 modulation. Modeling and simulation with the Lumerical software shows that compared with traditional TWMZM, AP-TWMZM has higher modulation efficiency and its linearity is as high as 0.948.