During the manufacturing process of harmonic reducers, if the accumulated pitch error is introduced among the flexible gear teeth and the rigid gear teeth, there will be dual period time-varying torsional stiffness fluctuations in the power transmission of harmonic reducers, resulting in dual period parameter vibration problems. In this paper, the double trigonometric series based on the combination frequency is introduced to approximate the forced vibration response of the dual periodic parameter system. By using harmonic balance operation, the recurrence formula of harmonic coefficient without time variable is obtained, and the three-dimensional matrix algebraic equation is formed. Using the matrix dimension reduction, the three-dimensional matrix algebraic equation is transformed into two-dimensional matrix algebraic equation, and the response harmonic coefficients have been solved. The research results have certain theoretical guidance significance for the research and engineering application in the forced response problem of dual periodic parameter vibration system.