Abstract: In this paper, the welding seam anti-corrosive applicator in-pipe robot for water injection pipeline with cement mortar liner in oil field is developed, firstly, its systematical structure and operation principle are introduced,then its polishing unit is analyzed thoroughly which features with compact structure,one motor with multi-functions to operation unit and perfect polishing function etc, finally the experiment shows the ideal effectiveness of the polishing unit, and it deserves to be recommended in engineering application.