Abstract: A method was brought forward to make the matrix become sparser because the memory space of the boundary element matrix is in direct proportion to the square of mesh number. An appropriate rule of sparsity is defined according to the characteristic of boundary element matrix at first. The matrix coefficients which are less than certain limit value are united into the adjacent elements. Afterward the matrix coefficient was evaluated as zero. And the matrix coefficient of partial adjacent elements can be incorporated into certain element by using this method. Then the boundary element matrix becomes a sparse one. By using this method, the memory space is reduced observably while guaranteeing the numerical simulation accuracy through numerical simulation.