As a light steel structure, the horizontal lateral resistance is an important parameter for the aseismic design of container house, while there is no reliable theoretical calculation formula. To provide reference for design and improve the theoretical system of corrugated skin structure, this paper deduced the calculation formula of lateral stiffness of container house. Different from the traditional skin structure using bolt connections, the side wall plate and steel frame of container house are connected by welding, the shear stress distribution and deformation composition in plane vary greatly, and the key point of solution is to analyze the diaphragm effect of corrugated plate under the welding boundary condition. Based on the energy theory and ABAQUS finite element simulation, the ideal lateral stiffness under uniformly horizontal distributed load and the local deformation under concentrated load at the top corner fitting were analyzed respectively. Then the calculation formula of lateral stiffness was derived, and it was verified by the experimental data. The results show that: under the concentrated load, the local deformation is much larger than the overall lateral displacement and the local effect ranges with the beam stiffness and the specifications of the container. In practical engineering, it is effective to increase the beam's effective sectional area or the amounts of connecting components for reducing the local deformation and improving the whole lateral stiffness.