High sensitivity of the intra-cavity absorption gas sensor (ICAGS) measurement network based on the erbium doped fiber ring laser was demonstrated. It is found that the maximum sensitivity can be obtained when the laser is operating near the threshold. Wavelength modulation/second harmonic detection technique was used to improve the performance of intra-cavity absorption measurement. A 1×N wavelength division multiplexer (WDM) with one channel corresponding to the absorption line of acetylene within the transmission window of 1530 nm was used in the WDM active sensing network. The system is demodulated by scanning the wavelength of a tunable filter. The minimum detectable gas concentration of 0.044 mol/m3 was achieved in measuring the 8.31 mol/m3 acetylene gas.