Abstract: Based on a node displacement interpolation formulation, the interface degrees of freedom reduction technique for a constrained substructure was developed in this paper, and the second transformation matrix was constructed for a constrain substructure modal synthesis method. The generalized degrees of freedom of the system were reduced effectively by using the second transformation matrix, thusly the calculating efficiency can be improved. The numerical examples demonstrate that the interface degrees of freedom reduction technique suggested in the paper is reasonable and effective. The influence on the calculation accuracy of the choice of interpolation nodes was discussed, and the numerical results show that the calculation accuracy is the highest if the interpolation nodes uniformly distribute on the interface. At last, the influence on the calculation accuracy of shape of interpolation areas was also studied, and the results indicate that calculation accuracy is higher when using the interpolation areas in the shape of rectangular than the case in the shape of triangle.