Abstract: DC will saturate the core of the current transformer (CT) and cause secondary current distortion, affecting the fairness of electric energy measurement. Two linear lines are used to express the excitation characteristics of the CT core, and combining DC and secondary fundamental current balance equations, the simple calculation formula of ratio difference and angle difference of measuring low-voltage CT under DC bias are derived. The relationship between CT error characteristics and DC, transformer ratio, and primary current is analyzed through numerical simulation. The DC bias experiment platform of CT was built, and the angle difference and ratio difference of three types of CT were measured under different working conditions. The correctness of the theoretical formula and analysis conclusion is verified by comparing it with the simulation calculation value. The research results can be used to quickly calculate the error value of CT under DC bias in engineering and provide a theoretical basis for evaluating the magnitude of DC impact on actual operating current transformers.