Mathematical model of infrared laser heterodyne gas analyser for extracting signal is described in this paper. It is proved that the detecting sensitivity of heterodyne gas analyser can reach up to 10-4ppb, about nine orders of magnitute higher than that of direct detection. The spectrum form of square-wave controlled laser, optical signal heterodyne detecting device and principle of electrical signal demodulation are considered with emphasis. The flow chart of the gas density signal is deduced, according to which the block diagram of controlled CO2 laser heterodyne gas analyser is dxawed.