A real-time, on-line gaseous detection system with high-resolution spectrum is built based on LabVIEW. It employs off-axis cavity enhanced absorption spectroscopy technology, and uses an external cavity diode laser (ECDL) as a laser light source. Two kinds of scan mode, one in roughness, the other in fineness for spectral measurement are achieved. A weak absorption spectrum of CO2 molecule as well as the relationship among the spectral intensity, line width and gaseous concentration is obtained at wave number of 6358.65 cm-1. The minimum detection sensitivity is 1.1×10-6 cm-1 for the entire system when using the 6358.65 cm-1 spectral line. Vibration-rotation spectra of CO2 molecule from experiment is basically consistent with the simulation results in the wave number range of 6450~6530 cm-1. Experimental results indicate that this system is not only feasibility but also has high detection sensitivity and spectral resolution, and meets the different spectral detection requirements of gas.