Taking the interaction between the laminated rubber bearing and reinforcement concrete column in the series isolation system into account, this paper derived the calculation formula of horizontal stiffness of the laminated rubber bearing and column and the series isolation system itself under the axial force and share force by the direct integration method, which were analyzed as the shear–bending model, and proposed its simplified calculation formula, and compared with the calculation formula presented by Kelly and mainly studied the influence of the typical parameters including the height ratio κ and the axial force factor σR on the components and the series isolation system, respectively. The results show that the horizontal stiffness decreases with increasing the typical parameters κ and σR, and the value of simplified calculation formula is closed to that of calculation formula derived of series isolation system in the certain range, and satisfies the engineering requirements .