Trace gas analyzers based on the tunable diode-laser absorption spectroscopy (TDLAS), combined with the long-path absorption cell, provide high sensitivity, high precision and fast response measurements of trace gases. The interferences caused by laser noise, electronic noise and optical niose, result in measurement error of the second harmonic signals. Characteristics of the second harmonic signal with TDLAS are analyzed. Mathematical model of the second harmonic signal is set up. Linear regression method is used to deal with the second harmonic signal of the measured trace gas using the signal of high concentration gas as the standard. The relationship between concentrations and corresponding correlation coefficients is also studied. Some measurements of CO and CO2 are presented, and the validity of the arithmetic is proved.