The initial stiffness of structures is an important parameter in engineering seismic design. Based on the quasi-static test of a frame structure which has embedded ALC slab, a mechanical model for infill frame structures is proposed and a formula for calculating the initial stiffness of structures is deduced according to the experimental phenomenon. In addition, five groups of 28 finite element models were established to study the effects of beam-to-column linear stiffness ratio, of the height-to-width ratio of the wall, of the elastic modulus of caulking material, of the elastic modulus and thickness of the wall on the stiffness contribution of filled wall. It is found that: the stiffness contribution of infilled wall is mainly affected by local compression, and its contribution is negatively correlated with the ratio of wall to frame lateral stiffness. On this basis, obtained through fitting was the relationship between the wall stiffness reduction coefficient aw
and the wall to frame lateral stiffness ratio. The comparison between the calculated results and the experimental results shows that the calculation formula proposed for calculating the initial stiffness of the infilled frame is reliable; can be applied in practice engineering design.