The invention provides a horizontal centering machine and relates to the field of seamless steel pipe production. The horizontal centering machine comprises a rack and a cone rod mounted on the rack. The cone rod is arranged horizontally and can reciprocate along the axis of the cone rod. A cone head is mounted at the front end of the cone rod. A drive device is connected to the rear end of the cone rod. The horizontal centering machine further comprises two clamp arms symmetrically arranged on the two sides of the cone rod. The clamp arms are rotationally mounted on the rack. The end, facing the rear end of the cone rod, of each clamp arm is a drive end. The other end, facing the front end of the cone rod, of each clamp arm is a work end. The drive ends of the two clamp arms are connected through an elastic element. The cone rod is further provided with a reducing portion protruding towards the drive ends. Each drive end is in abutted contact with the outer wall of the cone rod. The two work ends are driven by the corresponding drive ends to be separated and combined. According to the horizontal centering machine, the clamping and centering continuous action is achieved by one drive device, and the production efficiency is improved.