A simplified calculation method considering the stiffness of ordinary floors was proposed in the design of the frame-core tube structure with outriggers, and the results of this method is compared with Finite Element calculating results. Parametric analysis was conducted to evaluate the influences of various factors, such as the number of ordinary floors, the stiffness ratio of ordinary floors, outrigger stiffness ratio, outrigger span ratio and height-tospan ratio. A mathematical formula to calculate the stiffness ratio of the equivalent core tube considering the stiffness of ordinary floors is proposed. And its influencing law on dynamic property of outrigger structures is also compared and analyzed. The results show that the simplified calculation model proposed in this paper can well repeat the results of Finite Element model , the error rate can be reduced by 20% compared with the case without considering the ordinary floor stiffness. The proposed stiffness ratio formula of equivalent core tube can well reflect the contribution of ordinary floors to the overall structure stiffness, and meet the accuracy requirements of engineering scheme design. Moreover, the fitted stiffness ratio calculation formula of equivalent core tube can significantly reduce the computa?tional effort of dynamic analysis and approximately estimate the dynamic characteristics, providing a practical analysis method and theoretical basis for the dynamic analysis of outrigger system.