Laser diodes (LD) are used extensively in fiber lasers as pump sources. The operating characteristic of laser diodes determines a requirement of constant current regulation, high stability and high current in the drive circuit. This paper gives a design of constant current source aiming at the drive circuit of fiber laser pump sources with 24 volt power supply. It is made up of Buck module and constant current module. The synchronous Buck module consists of the chip LT3743, MOSFET Si7848BDP and RJK0330DPB, with the advantages of high-current output, low dissipation power and high power-supply efficiency. Circuit simulation results are given by the simulation software LTspice. It is shown that the Buck module satisfies the need of the circuit.