The Horizontal Seismic Decrease Coefficient （HSDC） is an important parameter in seismic isolated design. This paper investigates the influencing factors of HSDC theoretically in the Two-Degree-of-Freedom equivalent vibration system， in which the stiffness ratio of superstructure to isolation layer （SRSI） is regarded as a basic variable. The equivalent converting coefficient of the superstructure was calculated by mode-shapes analysis. The internal force and HSDC were both derived and expressed in SRSI， and then the influencing factors of HSDC were analyzed. The limits of SRSI， required for the seismic isolation structure to meet the requirements of HSDC， were calculated according to the design codes. The fixed curve of HSDC was found and defined as the Datum Curve， as well as the concept of Datum Points in HSDC was further proposed for design. The results show that： HSDC is determined by SRSI and the damping ratio of the isolation layer， and furthermore， HSDC decreases with the increase of SRSI and the damping ratio of the isolation layer； the HSDC is lower than 0.5 or 0.4 when SRSI is greater than 4.5 or 8， respectively； the HSDC’s expression gives accurate estimates when comparing with 5 example projects. The results are arranged into tables for reference.