The invention relates to a term extraction method, in particular to a method for extracting internal force without changing interface connection stiffness. The method comprises the steps of 1, judging a calculation amount: when a calculation scale is relatively small, executing the step 2, otherwise, executing the step 3; 2, extracting the internal force by a GPFORCE method: calculating the internal force, and then ending the method; and 3, calculating the internal force based on physical information of an interface: calculating an overall stiffness matrix, calculating a physical displacement matrix, calculating node force matrixes, and combining the node force matrixes into the internal force calculated by the specific interface. The method has the effects of high universality and high efficiency.