To achieve full range and high-precision measurement of CH₄ gas, a laser CH₄ sensor based on tunable diode laser absorption spectroscopy (TDLAS) technology has been developed, which integrates wavelength modulation spectroscopy first harmonic normalization second harmonic (WMS-2f/1f) technology and direct absorption spectroscopy (DAS) technology. The sensor utilizes a distributed feedback laser with a central wavelength of 1653.72 nm as the detection light source, and realizes real-time inversion of CH₄ volume fraction through a built-in microcontroller unit main control chip. A fusion algorithm of WMS and DAS inversion volume fraction data based on fuzzy control S-type membership function is proposed. Minimum root mean square error (RMSE) verification is performed on ten similar sensors, and the optimal fuzzy interval is determined to be in the range of 0.024‒0.038. Through comparison, this algorithm has effectively reduced the root mean square error of the system compared to the traditional full range measurement method. The test results demonstrate that the remeasurement error range of the full range test of the sensor is -5.28%‒3.67%, indicating that the sensor has high accuracy for the full range measurement. The detection limit of the sensor is evaluated at 3.04×10^-5 using a 3×10^-4 2f/1f measurement signal, combined with the optical path length of 0.07 m, and the minimum detectable integral volume fraction in 1 m optical path is 2.13×10^-6. The 0.01 volume fraction gas is measured for a long time and the Gaussian distribution of its inversion concentration is calculated. The maximum statistical value is in the range of 0.997×10^-2‒1.002×10^-2, and the half width at half maximum of the Gaussian fitting curve is 1.43×10^-4.