This paper introduces the working principle of a voltage-controlled current source circuit, which is an improved version of the traditional Howland current source circuit. It employs a differential operational amplifier (op-amp) circuit structure to construct a voltage-to-current conversion circuit. By using an FPGA to control the DAC output to generate different voltages, the circuit achieves current output ranging from 4 mA to 24 mA. The paper analyzes the impact of factors such as the op-amp, DAC, and resistor network on current output accuracy, providing guidance for component selection and circuit design. A compensation method using linear fitting is proposed, effectively improving current output accuracy. The constant current source circuit built using the SGM8249 operational amplifier was tested under temperature conditions ranging from -30℃ to 70℃. Within the current output range of 4 mA to 24 mA, the maximum error did not exceed 0.03 mA, and the relative full-scale error was less than 0.15%, meeting the requirement of current output accuracy less than 0.3% in actual engineering applications.