A new method of tracing precipitation isoline is introduced in term of the character of rain spot and iso-line. The method, firstly, grids the precipitation amount by conicoid, then judges and traces isoline and brings forward a way of how to avoid connection uncertainty of isoline. Lastly, jumping-off and end point of opening-curve,and close isoline were judged. The method improves the precision of the precipitation isoline and is important to count the surface precipitation of the drainage area.