A fully differential TIA circuit and optically isolated IGBT drive system was designed in a 0.18 μm BCD IC process. The effects of common mode transient noise on the optical isolation were analyzed, and a fully differential TIA structure for improving common mode transient immunity was proposed. The differential input terminals of TIA were connected with a light shielded PD and a normal PD respectively. The level of differential output terminal of TIA was compared. In this way, only one input of TIA could receive the optical signal to generate the differential mode gain, but the coupling of common mode transient interference in the isolation layer could be transmitted to the two inputs of TIA, so the interference effect of common mode transient would be weakened by CMRR. The narrow pulse filter circuit was added to filter out the short pulse error signal caused by common mode transient interference and further improve CMTI. The simulation results show that the CMRR is 105.4 dB, and the CMTI is 325 kV/μs.