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.