The design of the fast kicker prototype magnet for China Spallation Neutron Source(CSNS) rapid cycling synchronous(RCS) extraction has been introduced. Single-turn structure of SNS in U.S. was adopted for magnet power supply research difficulties and operation reliability. The choice for material of iron core, all the structure design and the method of collimation for the magnet were also described. Simulation results of Opera show that when the length of the core is 220 mm, the effective magnetic length is about 302 mm and the maximal magnetic density is around 0.215 T at four interior angles. Corresponding field uniformity is within 0.8% in the area of 133 mm×200 mm. Theoretical analysis and program simulation of Opera have verified the feasibility of magnet basic structure.