Abstract: A type of four-link terrain perceptional mechanism, TTSMS, is designed using the motion principle of four-link lever mechanism and combing the characteristics of terrain environment of urban area and building. Describing the principle and method of sensing terrain by defining the convex-shape obstacle. It is capable of sensing the terrain rapidly and reliably, not almost suffering from disturbance of real world，and measuring the type of obstacles directly. The key problem of perception of obstacle in real time for mobile robot moving in three-dimensional terrain enuironment is resolved.

Key words: Terrain tentative sensing, mechanism/sensor, Mobile robot, Obstacle negotiation, Terrain perception