A moving interface reconstructing algorithm keeping volumed fraction of fluid in the volume of fluids (VOF) method is presented. The moving interface is approximated by a diagonal line of a grid, and two diagonal lines located in adjacent grids intersect on a public side. By keeping constant volume fraction of fluid in each grid, nonlinear equations are built on boundary grids, location of intersecting points of moving-interfaces with grid sides are decided by solving equations. The shape of moving-interfaces is reconstructed. The method simulates accurately moving interfaces with fewer cusps. Its availability and efficiency are demonstrated.