The laser driving circuit is an important part of the transmitter in the optical communication system. In order to meet the needs of high-speed electric/optical conversion, an externally modulated laser(EML) driving circuit is designed based on the 28 nmcomplementary metal oxide semiconductor(CMOS) process. The whole driving circuit adopts a five-stage cascade architecture, including variable gain amplifier (VGA) and two-stage continuous time linear equalizer (CTLE), preamplifier and output stage. The driving circuit adopts a new complementary amplification and cascode structure, the input of the drive circuit is a 50 Gb/s fourth-order pulse amplitude modulation(PAM4) signal. The simulation results show that the small signal bandwidth of the driving circuit is 27 GHz. When the output swing is 1 VPP at 12.5 GHz and 0.22 VPP sinusoidal input, the total harmonic distortion(THD) is 2.21%. The voltage gain range is 13.98~20.24 dB, and the total power consumption is 340 mW.