Abstract: GS-STEP is a STEP toolkit that adapts to open environment. To meet the needs of flexibility in open CIMS environment, authors took system independence, protocol independence and hierarchical modularization as the development strategy. With this strategy GS-STEP could be conveniently embedded in all kinds of CAx systems, and each functional unit could be used independently as software toolkit or development platform, which embodies the thought of 'Plug and Play' and the principle of extensibility. As a platform for information integration, GS-STEP has been successfully used in the integrated CIMS environment of GS-ICCC——an integrated CAD/CAPP/CAM software developed by the authors too. This paper addresses the design and implementation of GS-STEP from the viewpoints of development strategy, system structure, data flow and function organization.