The invention discloses an online treatment device for large bar rolled piece bend. The online treatment device comprises an overall framework, a horizontal straightening roller pair and a vertical straightening roller pair, the horizontal straightening roller pair is arranged on the left side and the right side of the overall framework, the vertical straightening roller pair is arranged on the upper side and the lower side of the overall framework, the horizontal straightening roller pair and the vertical straightening roller pair respectively form a horizontal straightening hole and a vertical straightening hole or jointly form a straightening hole, and roller gap adjustment driving devices for adjusting the positions of the horizontal straightening roller pair and the vertical straightening roller pair are further arranged on the overall framework. After the online treatment device is mounted on a large bar continuous mill finish rolling frame, a rolled piece is treated online by the horizontal straightening roller pair and the vertical straightening roller pair, the size of the straightening hole can be adjusted by the roller gap adjustment driving devices to adapt to bars with different specifications, and the online treatment device successfully solves the problems that rolled pieces still bend after cooling collection due to bending of finished product pass rolled pieces in the rolling process of a large bar production line, manpower is wasted in treatment and production cost is increased.