Based on the fundamental frequency absorption peak at 3.37μm of propane butane, a propane butane gas detection system based on wavelength modulation technology was developed by using a continuous interband cascade laser(ICL) with corresponding center wavelength and an absorption cell with a long optical path.In order to automatically determine the center value of sawtooth wave in the wavelength modulation process, based on the linear relationship between current and output frequency, the direct absorption spectrum was obtained by uniformly increasing the driving current of the laser.The Lorenz profile fitting algorithm was used to determine the current corresponding to the absorption peak so that the sawtooth output light can cover the absorption peak with its center. The experimental results show the sawtooth wave with the fitting result of 31.94mA as the central value, after the high-frequency sinusoidal modulated wave with the superposition frequency of 20000Hz and amplitude of 0.03V, the second harmonic peak exceeds 100mV. The whole process of the algorithm does not depend on precise optical devices such as gas and F-P etalon.