In this paper,a line integration method(LIM)is proposed for treatment of domain integrals in boundary element method.The method is based on Divergence theorem and the domain integrals can be translated into line integrals on straight lines finally.The integral lines are created from the boundary elements and the line integration method is a boundary-only discretization method.To further simplify the line integrals and improve the accuracy,background cell structure is applied to cut the initial integral lines into sub-lines.Applying the method directly is time-consuming since the complexity of the CPU time is O(NM),where N and M are the numbers of nodes and lines,respectively.To overcome this problem,fast multipole method is further applied in line integration method for large scale computation.Numerical examples have shown the accuracy and efficiency of the fast multipole line integration method.