Abstract: In order to realize the trace gas concentration on-line measurement and analysis，a multi-type gas concentration on-line measurement system based on fiber-circled cavity ring-down method was designed and developed. Based on the nonlinear optical cavity ring-down technology, the experimental analysis of the gas concentration in-situ measurement with such system was performed. The gas-to-light absorption characteristics of the same gas with different concentrations changed with time and their relationship was given. The resolution of concentration measurement reaches 1ppb. The experiment proves that the system can be used to measure trace gas concentration effectively.