A yield monitoring software was developed by VB programming language could be applied to combine harvester based on weighing. The data collected by yield monitor system could be displayed and saved by the software in real time. The software could get the harvesting total mass and the area, and draw the grain flow diagram by computing the grain flow data. It can also convert the latitude and longitude data received from GPS into Gauss coordinate, and then draw GPS locus diagram in rectangular plane coordinate system. Finally, the software can draw the yield map automatically by using the grain flow data and GPS locus data at the same time. During the process of drawing the map, the map will move itself far away from the border of interface to ensure the real time curve displaying normally before the curve arrives the border of interface. After finishing drawing, the map could be dragged in the interface freely to check the whole map. The yield monitor precision was less than 2% in laboratory and less than 3% in field.