The stability of the driving current directly determines the stability of the output wavelength,which in turn affects the accuracy of the detection. In order to meet the demand for detection accuracy and constant current output in gas concentration detection by the Cavity ring down spectroscopy (CRDS),a constant current drive circuit with digital and analog circuits was designed. STC90C51 is used as the main control chip to complete the scanning keyboard,DA conversion. The analog circuit is mainly composed of negative feedback operation amplification,high-precision CMOS tube and feedback resistance to complete the voltage to current conversion,current output to the butterfly semiconductor laser,and realize the constant current drive of the butterfly semiconductor laser. The test results show that the drive current of the circuit is adjustable within the range of 0~300 mA,and the drive current error is less than ±0.003 mA,which meets the system′s error accuracy requirements for constant current drive of ±0.005 mA.