The laser diode which is the key of external cavity laser diode (ECLD) is a very tender part. It is easily damaged by the impact of the voltage,current and static when its assembly and running. To avoid the damage,the feedback constant current theory,low startup theory,low close up theory and MOS effect transistor speciality are applied to realize a new method of high-resolution constant current circuit,after some traditional methods used for reference. The new method is applied for the high resolutional external cavity semiconducter laser diode of the 1550nm wavelength to drive the current constantly. The resolution of drive circuit can be achieve in high level and be above 0.01%. The project is simulated by Protel DXP tool,designed by the PCB circuit,and applied to the laser driver circuit system,which has proved the constant current model can avoid the damage for drive current fluctuation availably.