In this paper, an approach for a high stable constant current source is presented, which is based on multiple power MOSFETs paralleling connection method, the maxim output of current source is 30A, and is continuously adjustable. The performance of the designed constant current source has been tested, the measurement results indicate that the stability of constant source is better than 5ppm when the current is 30A. The working principle of the designed constant current circuit is analyzed in detail, and the selection of key components is explained clearly. Using high precision reference voltage source and current series negative feedback based on high-power precision sample resistor to increase the stability of system. In order to realize the high stable current, multiple MOSFETs in paralleling connection are used as regulator, and systematic analysis on factors affecting MOSFETs in paralleling work are presented as well. The constant current source we designed has a certain reference value for the design of DC current source.