A kind of automotive switch power supply, which is based on LM25116, has been designed, with 24 V its input DC voltage, and 12 V its output DC voltage. By adopting BUCK circuit topological structure, the system substitutes MOS tubes with lower impedance for the traditional diodes, thus achieving the synchronous rectification and improving the efficiency of the power output. A thorough analysis has been made of the working principles of the main circuit, the control and drive circuit. The experimental results show that a stable output voltage can be achieved, with an output efficiency even higher than 90%, an indication that its function and performance meet the design requirements.