During the process of setting up a carbon monoxide infrared laser generator, it's necessary to analyze the gaseous components produced in laser cavity under reduced pressure for understanding its charging regularity. In this paper a self-made gas chromatograph for analysis of some special gas mixtures is described. This instrument can analyze O2, N2, CO (with normal contents) and trace CO2 simultaneously below atmospheric pressure. The gas sample below atmospheric pressure was taken into a small piston cylinder and then compressed by pushing the piston for injection. The injected sample was carried by He into two parallel chromatographic columns and detected separately. The gases with normal contents were detected by a thermalconductivity detector (TCD) and the trace CO2 was detected by a He ionization detector (HeID). The detection sensitivity of CO2 is in the order of magnitude of 10(-6) V/V. The experimental results show that this instrument with advantages of simple design, easy operation and good stability is suitable for on-line analysis of gas samples below atmospheric pressure. We have successfully analyzed on-line the gaseous components produced in the laser cavity by using this equipment. The technique described here can be used for complete analysis of gas samples below atmospheric pressure in many other fields.