This paper studies the basic principles of finite element method, then researches on the core algorithm of finite element method, according to Saint-Venant principle, introduce the concept of composite element, proves that if the element is composite element, the original integration is still able to calculate. Based on the feature that numerical integration solution is in fact the Gaussian integration points are calculated, we endow element internal with different material property values according to the different location artificially. This is the efficient and local accuracy decrease algorithm. The algorithm is able to simplify the work of mesh generation, improve the overall computing efficiency, is especially suitable for underground engineering, and we can quickly get the overall characteristics of the structure that we are most concerned about. Finally, builds the actual examples of underground engineering, uses the finite element software Ansys and the efficient and local accuracy decrease algorithm program to calculate the actual examples respectively, compares the results between them, expounds and set the analysis theory and relevant principles, makes conclusion: the efficient and local accuracy decrease algorithm can simplify mesh generation, improve the overall computing efficiency; and it can be used in underground engineering very well.