Several key optical components in a reflective fiber optic current sensor (R-FOCS) show error characteristics, which induces linearly polarized light wave in polarization maintaining (PM) fiber and circularly polarized light wave in sensing fiber to be an elliptically polarized light wave. Therefore, nonreciprocal phase shift induced by magnetic field of the current is interrupted by the wrong polarization state. Focused on optical polarization error in R-FOCS, we analyze the characteristics of polarization errors associated with optical components in R-FOCS theoretically, and demonstrate the methods to build optical polarization error models by using Poincaré sphere and Jones matrix, respectively. Then, based on a model of polarization states in R-FOCS, we theoretically investigate the effect of several main error factors on optical polarization characteristics, including polarizing performance of polarizer, phase delay in quarter-wave retarder, splice angular between quarter-wave retarder and PM fiber, birefringence in sensing head and mirror reflection phase shift error. Finally, we quantify a nonreciprocal phase shift to be detected in R-FOCS with optical polarization error respectively, and propose measures.