A new type of in-pipe mobile robot was designed and developed on the basis of wheel in-pipe robot and crawler in-pipe robot. The three sets of driving wheels circumferentially 120° apart in the cross section, both front and rear driving wheels are distributed on the same parallelogram mechanism. The driving motor drives the three sets of driving wheels by worm couple, the regulating motor makes the three sets of driving wheels push against the pipe inwall with stable and adequate pressing force by the ball screw pair and pressure sensor, so the in-pipe robot can provide adequate and stable traction force. The robot mechanism is simple and small in size and work reliably. It is particularly suitable to the pipe with diameter 400～650 mm.