The isolation design method, which is proposed in the Standard for Seismic Isolation Design of Building, is an integrated design method based on the equivalent linearization of the complex mode decomposition response spectrum. However, this method does not specify how to determine the period reduction coefficient of the isolation structure. Therefore, according to the stiffness series relationship, a simplified calculation formula for the period reduction coefficient of the equivalent single-degree-of-freedom isolation system was derived, and the influencing factors of the period reduction coefficient of isolation structures were discussed in this paper. Numerical finite element models with different filling materials and rates were established, and the accuracy of the simplified formula was verified. Results show that a functional relationship exists between the period reduction coefficient of the isolation structure, the nominal period ratio, and the period reduction coefficient of the non-isolation structure. As the filling rate increases, the period reduction coefficient of the isolation structure decreases. For the same filling rate, the period reduction coefficient of the isolated structure decreases with the increase in the elastic modulus of the filling materials and increases with the wall opening. The range of the period reduction coefficient of the isolation structure is 0.915-0.965. The deviation between the numerical simulation and theoretical analysis results is less than 3%, thereby proving the accuracy of the simplified formula. The simplified calculation method of the period reduction coefficient has a certain guiding importance for the design of isolation engineering.