Abstract: In this paper a similar-topological graph（STG）which has entire topological feature and individual property is presented to describe pipe nets with netted structure.A fast efficient transfor- mation method is offered to generate an expanded triangle net（ETN）from STG.An efficient algor- ithm based on the transformation method for drawing STG contour lines is given.All the experimental results obtained correspond with both our demands and the real condition of the pipe net and seem satisfactory.