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In this paper, we derived the expression for the instantaneous intensity based on numerical simulation and practical experiments in tunable diode laser absorption spectroscopy, and determined the new expression of the harmonic signals in wavelength modulation spectroscopy, then we analyzed the influence to the harmonic signals caused by the phase shifts between the detected and reference signals. The transition of CO2 at 6527.64 cm–1 was selected for the practical experiments, and the experimental results indicated that, the amplitude of laser intensity modulation was invariant during the scanning process, which did not change with the average scanning laser intensity. What’s more, the amplitude and phase of the harmonic signals were influenced by the phase shifts between the detected and reference signals, which cannot be ignored in gas concentration and temperature measurements.