Many existing methods of obtaining Optically Coupled Isolation Amplifier (OCIA) frequency characteristics are based on the estimation of rated parameters. The internal component parameters change after a long period of operation, and the internal components of the transformer no longer work with the rated parameters. As a result, the frequency characteristics obtained based on the rated parameters are no longer accurate. To solve this problem, this paper proposes a new OCIA wide frequency domain modeling method without relying on OCIA parameters. The OCIA transfer function values at different frequencies are obtained by measurement, and the error function is constructed based on the rational approximation fitting principle. Then the particle swarm optimization algorithm is used to optimize the error function, and obtain the expression of the fitting transfer function. The accuracy of the algorithm is studied in different frequency sampling ranges and noise levels. Simulation and measured data verify that the proposed method has high accuracy and good anti-noise ability in the wide frequency domain. This work is supported by the Science and Technology Project of the Headquarters of State Grid Corporation of China (No. 5200-201926078A).