The new-generation standards of Geometrical Product Specifications (GPS) take the measurement and verification of manufactured parts according to the duality principle. The Specification Surface Model (SSM) is a simulation model of the actual manufactured part which plays a decisive role in the process of duality verification. The existing methods, however, have not been found to carry out much related research on the generation of SSM. This paper presents a method to generate the SSM for the characteristic of planar feature with flatness under the standards of GPS. The mathematical model of the flatness tolerance is firstly established. Secondly, in order to get the simulation points to simulate the actual characteristics of the manufactured parts, both of the functional requirements of the parts and the influences of the manufacturing process are considered. And then the simulation points under the normal distribution are generated within their tolerance zone by computer. By using the approximation method of the least-squares SSM is then obtained. Finally, an example is used to illustrate the proposed method.