There are various computer-aided methods and softwares for drawing precipitation isolines, besides manual methods. We propose a new method complete drawing the precipitation isolines with grid method on computer. Comparing with the traditional method, it is more objective. We divided a watershed into a finite number of units. According to the precipitation value of all the precipitation stations and their location, we calculated all the units of precipitation with a certain numerical methods. Then, we used a certain numerical unit area boundaries as isolines. Finally, we completed drawing the precipitation isolines of the watershed.