Abstract: While computing radar cross section(RCS),surfaces of objects were primarily triangulated, which was called prepositive disposal. The length of the edge of the triangle was generally 0.1 wavelength(in general one wavelength is 3 cm).The shape of aircrafts was complex , aircrafts’ geometric model contained many trimmed surfaces, the relationship between trimmed surfaces was complex. Meanwhile surfaces were triangulated into many triangles with the edge of 3 mm, therefore the number of triangles was huge. All these resulted in difficulties in the prepositive disposal. An algorithm of trimmed surfaces triangulation was put forward. Using the characteristic of aircraft model and satisfying the approach error the surface was dispersed by isoparametric curves, the curves were dispersed into points at the given length and the meshes were composed of points on two adjacent isoparameteric curves. According to the position relation of meshes and the trimmed area meshes which were out of the trimmed area were trimmed. Then meshes on the surfaces boundaries were converted into being consistent. Triangulation of trimmed surfaces can be realized.