The main functional modules of In-situ gas analyzer ITLA-1000xx (In-situ laser gas analyzer) include a transmitter, receiver, reference unit, and measurement unit. The transmitter drives the tunable diode to emit a laser of certain wavelengths, which passes through the gas cell, then reaches the receiver. The receiver performs signal processing to obtain the Second harmonic signal, then calculates the processing to obtain a second harmonic signal, then calculates the concentration according to the relationship between the second harmonic signal and the gas concentration.