Abstract: This project is a large box mat foundation （long 686 m） with complex load, and there isn't mature experience in computation of its deformation. In course of construction, the design theory of the foundation and the field measurement of its behavious are concerned by the project organization and the law of deformation of the foundation are found out. After the engineering put into production in recent years the results of the field measurement proved that the theoretical calculation is consistent with engineering practiae.In this paper some brief explanation on the assumption of the theoretical calculation are given, such as consideration of the interaction of the earthground with the foundation and the correction of rigidity in the settlement computation, adopting the assumption of approximate calculation in parts, the application of the theory of Winkler elastic foundation and the analysis of the calculation of all length foundation etc.