In this paper,the new CW laser gas concentration measurement system based on the cavity ring-down method was introduced,and the analysis to the principles and functions of the main devices were done during the designing progress.Then,the ring-down times of seven certain light in seven corresponding gases（with the same concentration of 100cm3/m3）were obtained,and the gas absorption characteristics in different concentrations were given.Also,the ring-down time-gas absorption coefficient-gas concentration relation and the loop loss-gas absorption coefficient-gas concentration relation were analyzed,all of which proposed a new method for the gas concentration in-situ measurement.