In geog raphical informat io n ( GI) draw ing , there ar e many random curves, like bo undary lines, reiver s, coast s and ro ads, etc. During the computer processing of GI, it is capacity-consumed to describe these curves t ruly , and the data ex chang ing between internal and external memory may be t ime-consumed. The method descr ibed in this paper used the broken line to define this kind of curv es, implement ing random pro cessing fo r ever y edg e of the broken line, and simulat ing the random cur ves. This method is helpful to cr eate the r andom curv es in hydr olog y and has been applied in the info rmat ion manag ement of w ater conservancy pro ject .