Abstract: Streamline construction is an important part of visualization for flow fields, it presents also the basis for constructing streamribbon, streamtubes and stream surfaces. Typical algorithm of streamline construction is mostly too complicated for general users. This paper presents a simplified model of decomposing the velocity in grid cell, identifying the outflow surface of fluid flow by discriminant conditions and tracing the streamline according to connection relations of cells. Our algorithm is verified with CFD computational result of passageways for large-size pump stations, the visualization effect is fine.