A gas detection system comprising a case having a hollow chamber, a gas input port, a gas output port, a radiation emitting device, and a photo detector. The gas input port may be disposed on the case for a test gas flowing into the chamber. The gas output port may be disposed on the case for the test gas flowing out of the chamber. The radiation emitting device may be disposed on the case and operated in a surface plasmonic mode or a waveguide mode for emitting a narrow bandwidth thermal radiation light source with multi-peak wavelengths into the chamber, wherein the multi-peak wavelengths may comprise a first absorption wavelength and a second absorption wavelength of the test gas. The photo detector may be disposed on the case for detecting light intensity of the light source passing through the chamber to determine the concentration of the test gas.