The invention provides an extensible robot for climbing an inner wall of a pipeline. The extensible robot comprises a middle connection mechanism, four extensible arms and two extensible mechanisms. Each extensible arm comprises a friction pad, an arm extensible motor, an arm extensible motor holder, an arm extensible pipe connection frame, an arm extensible lead screw, two outer arm extensible pipes and two inner arm extensible pipes. Each extensible mechanism is composed of an arm connection frame, an extensible motor, an extensible motor holder, an extensible pipe connection frame, an extensible lead screw, two outer extensible pipes and two inner extensible pipes. The middle connection mechanism comprises a center rotary frame I, a center rotary frame II, a steering motor I, a steering motor II and a center joint cross. The middle connection mechanism is fixedly connected with the extensible motor holders of the two extensible mechanisms through the center rotary frame I and the center rotary frame II. The arm connection frame of each extensible mechanism is fixedly connected with the arm extensible motor holder of the corresponding extensible arm. Each arm extensible motor holder is symmetrically provided with two extensible arms. The fixed position and the spatial two-freedom-degree rotation of the robot in the pipeline can be achieved.