Nowadays, as the development of economy and technology, more and more electric devices need a DC voltage power source to supply a constant current. The Magnetic controllable transformer is also use a DC control module to provide a stable and reliable DC current. By a TMS320F2812 DSP chip as controller, this paper presents a design of DC constant current power supply based on the BUCK topology. A power prototype was set up in the laboratory, test results proved that the design is well applied to a magnetic controllable transformer.