Ferrocement-LGS (Light Gauge Steel) composite construction is recently proposed novel form of construction as a substitute to traditional RCC framed construction. In this construction form, walls of the building behave more or less like a shear wall in resisting in-plane lateral loads. The present paper attempts to propose a simplified mathematical model for obtaining response of composite wall under in plane lateral loading. As first part of present work, in-plane force deformation behavior of Ferrocement-LGS composite wall panel is predicted through FEM analysis (ANSYS). This force deformation curve was employed to estimate equivalent spring parameters of simplified mathematical model (ETABS) of composite walls. The simplified mathematical model saves significant computational efforts in predicting the in-plane response compared to the traditional FEM model without compromising the accuracy.