Abstract: A tri-axial differential drive unit composed of a tri-axial differential mechanism and a pipe diameter adapting mechanism is designed for in-pipe robot,and both differential property and mechanics property in straight and elbow pipes are analyzed theoretically.In straight pipe,the drive unit doesn't perform tri-axial differential operation;in elbow pipe,it realizes autonomous differential function with pure wheel rolling and without parasite power according to the topological environment of internal pipe.The designed tri-axial differential drive unit of in-pipe robot has laid a foundation for theoretical research of differential in-pipe robot with mechanical adaptability.