Rotatable retarder fixed polarizer (RRFP) Stokes polarimeters are widely used to detect the state of polarization of light. The orientation error of the retarder is one of important error sources of RRFP Stokes polarimeters. In order to investigate the effect of the orientation errors of retarder on measurement results, a new analytical model is proposed to express the polarimetric error resulted from it, and a methods based on covariance matrix is used to express the polarimetric error. In addition, according to the analytical error model, the optimal polarimeter configuration is obtained. During derivation, we suppose that the angular orientation errors of the retarder obey a uniform Gauss distribution. Based on the analytical model derived, it can be concluded that (1) the measurement errors induced by the angular orientation error of the retarder is inversely proportional to N; (2) the measurement errors are independent to the incident intensity s0, but seriously depend on the incident state of polarization (s1, s2, s3) and the retardance of the retarder δ; (3) the retardance ranging from 10322° to 11613°, whihc results in the minimum measurement errors induced by the orientation errors of the retarder. Finally, simulation is performed to prove that these analytical results derived agree well with simulation results.