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