Abstract: The all-optical-fiber current transformer, known for its high safety, resistance to electromagnetic interference, and wide frequency response range, has been widely used in DC current measurement. However, its measurement accuracy is affected by the misalignment angle of the optical fiber sensing loop during fusion splicing. In this paper, a mathematical model for optical fiber current sensing is established based on the coupled mode equations and the Jones matrix theory, and the temperature characteristics and nonlinearity error of all-optical-fiber current transformer with fusion splicing errors are analyzed. A method is proposed to calculate the measurement error of the all-optical-fiber current transformer over the entire current range after on-site secondary fusion splicing based on the actual distribution characteristics of fusion splicing errors. The theoretically calculated deviations of the all-optical-fiber current transformer match the measured values over the entire current range. The nonlinearity error assessment method presented in this paper can determine whether the all-optical-fiber current transformer meets measurement accuracy requirements after on-site fusion splicing on condition that a full current range test is not conducted, thus measurement accuracy can be effectively ensured after the secondary fusion splicing of the all-optical fiber current transformer.