The invention provides a hydraulic device and a bend straightening machine with the hydraulic device. The hydraulic device comprises a hydraulic oil cylinder and a reversing valve. The hydraulic oil cylinder comprises a cylinder body, a piston and a piston rod. The cylinder body is of a hollow structure, and a first oil path channel and a second oil path channel are arranged at the bottom of the cylinder body. The piston is arranged in the cylinder body and divides space in the cylinder body into a first cavity and a second cavity. The first cavity is located between the bottom of the cylinder body and the piston. The piston rod is connected to the piston. The second oil path channel penetrates the piston and extends into the piston rod. The second oil path channel comprises a longitudinal channel and a transverse channel. The longitudinal channel extends in the axial direction of the piston rod, and the transverse channel communicates with the longitudinal channel and the second cavity. The reversing valve is connected with the first oil path channel and the second oil path channel, two oil path cycles opposite in direction are provided for the hydraulic oil cylinder, and the piston rod is driven to do reciprocating motion.