A laser scanning device installed at the end of the redundant manipulator is designed, in order to solve the problem of autonomous scanning and model reconstruction for unknown variable cross-section pipe. Firstly, the chosen laser range sensor and the scanner head structure which can rotate 360 degrees are described. Secondly, measurement model is built and a cross-section intersection based forward scanning method (CIFSM) for unknown variable cross-section pipe with our laser scanning device is put forward. Thirdly, a hypothetical boundary based obstacle avoidance scanning path planning algorithm (HBSPPA) is proposed. In scanning experiments, two representative cross-section shapes can both be scanned autonomously and rebuilt accurately in coordinate system of redundant manipulator. The results demonstrate that the proposed device and method are effective and practical.