A novel method for laser frequency locking and intensity normalization in wavelength modulation spectroscopy (WMS)-based gas sensor system is reported. The center spacing between two second harmonic peaks demodulated from the rising and falling edges of a scanning triangular wave (for wavelength scan) is employed as a frequency locking reference. Amplitude of the directly acquired sine signal (for wavelength modulation) in the spectral region far away from the absorption feature is employed as an intensity normalization reference. A 50 ppm CH4:N-2 sample sealed in a multi-pass cell at 1 atm was employed as the target analyte for demonstration. The frequency locking significantly improves measurement accuracy. and the introduced intensity normalization method realized a similar to 3 times SNR improvement as compared to the commonly used 1f normalization method under frequency locking conditions. A minimum measurement precision of similar to 2.5 ppbv was achieved with a normalized noise equivalent absorption coefficient of 1.8 x 10(-9)cm(-1)Hz(-1/2). (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement