The invention relates to a distributed infrared laser multi-parameter gas online detection system. The system comprises a light source driving module, a receiving optical fiber, an absorbing air chamber, a sending optical fiber, a photovoltaic conversion and signal processing module, a plurality of laser light sources and a light source conversion module, wherein the laser light sources have different central wavelengths and are switched by virtue of the light source conversion module, and therefore, only the light emitted by one laser light source can be focused into the receiving optical fiber after being collimated by one group of coupling lens; and the receiving optical fiber is connected with one end of the absorbing air chamber, the other end of the absorbing air chamber is connected with the sending optical fiber, and light waves transmitted by the sending optical fiber are output to the photovoltaic conversion and signal processing module after being collimated by the other group of the coupling lens, so that a second harmonic component of detected gas is detected, and the concentration of the detected gas is calculated. According to the scheme, the problems of long detection period and low efficiency in the measurement of a single parameter in the prior art are solved.