In order to provide stable and adjustable current source for portable systems, this paper designs a digitally controlled constant current source. According to the input of independent key, the output of the microcontroller digital volume controls digital-to-analog converter by Voltage-Current conversion module to get the constant current output. The output module consists of the operational amplifier and the power transistor TIP122, which is used for voltage-current conversion. At the same time, the value of output current is obtained by sampling resistor to convert the real-time acquisition and monitor the output of constant current source. The experimental results show that the system output ranges from 0 mA to 240 mA. The value of constant current can be controlled by stepper key, and the measurement error ratio can meet the requirement of design.