In order to improve the signaltonoise ratio of the measurement, a constant current source of high stability is usually required, and to extent the measuring range, the constant current source can be used for adjusting and controlling, Therefore, this paper presents a highly stable constant current source method. Through the differential amplification and feedback of the voltage on the sampling resistor, on the one hand the common mode noise can be reduced and the accuracy of feedback can be improved, on the other hand also the heat consumption of the sampling resistor can be reduced, these aspects provide the guarantee for the constant current source of long time stability, in addition, the constant current source can be linearly adjusted by a DAC. After a long period of testing, the output ripple of the current is less than 0.1mA, and under different voltage the error of theory and actual in about one thousandth. So the constant current source is of high stability, and high control linearity.