Proton beam is an effective means of radiotherapy for tumors. During the transmission of the proton beam from the accelerator to the treatment head, the direction of the beam is usually changed by the magnetic field generated by dipole magnet. A variety of magnets are arranged in a radiotherapy system using a rotating gantry for proton beam transmission. The 90 degree magnet is placed above the treatment head that move with the rotation of the gantry and concentric with the rotating gantry. The iron core of the magnet is often stacked with silicon steel sheets with high magnetic permeability and low loss. The outer coil is wound with oxygen-free copper conductor with water cooling in the middle. This article will describe the details of design and manufacturing process of a 90-degree bending magnet on the proton therapy rotating gantry.