Wavelength modulation spectroscopy(WMS) is a high sensitive spectral detection technique, and a precise signal analysis model for the typical application is the basis for system design and optimization. A theoretical description based on Fourier analysis of tunable diode laser wavelength modulation spectroscopy technique is given. The characteristics of WMS harmonics of tunable diode laser spectroscopy under sinusoidal frequency modulated are analyzed, and the influences of laser amplitude modulation and etalon effect of system various optical surfaces on the harmonic signals are studied. The formalism is not confined by the modulation amplitude, and can quantitatively elucidate WMS analysis and background signals under arbitrary situations.