The utility model discloses a flange straining device, including setting element, cooperation protection piece and the tensioning means on relative another flange back bolt hole of cooperation on a flange back bolt hole, tensioning means is including tightening up the piece and can receiving and draw tighten up the piece that pulls on the piece, pull the one end of piece and pass one in proper order flange and setting element and location are in another is passed in proper order in one side of setting element, the other end flange and protection piece connect tighten up on the piece. The utility model discloses a flange straining device, simple structure, economical and practical, wherein through setting element and protection piece coordinate respectively double -phase to the flange carry on the back the surface mutually on, draw the piece and pass setting element and the protection piece of two flange bolt holes on and, receive a dragline and draw the piece through tightening up, taut with two flanges, the simple operation is high -efficient, be applicable to the butt joint take -up of flange between the inflation is bent under water.