A palm-sized laser spectrometer has been developed for detecting trace gases based on tunable diode laser absorption spectroscopy (TDLAS) in combination with a novel double-layer toroidal cell. With the benefit of a home-made electronic system and compact optical design, the physical dimension of the sensor is minimized into 24×15×16 cm3. A toroidal absorption cell, with 84 reflections in 2 layers for an effective optical path length of 8.35 m, was used to enhance the absorption signals of gaseous species. A home-made electronic system was designed for implementing distributed feedback (DFB) diode laser controller, analog lock-in amplifier, data acquisition and communication. Calibration-free scanned wavelength modulation spectroscopy was employed to determine the concentration of the gas and reduce the random fluctuations from electronical noise and mechanical vibration. The measurement of CH4 in ambient air was demonstrated using a DFB laser at 1.653 μm. The rise time and fall time for renewing the gas mixture are approximately 16 s and 14 s, respectively. Vibration and temperature tests have been carried out for verifying the performance of the spectrometer, and standard deviations of 0.38 ppm and 0.11 ppm for 20 ppm CH4 at different vibration frequencies and temperatures, respectively, have been determined. According to the Allan deviation analysis, the minimum detection limit for CH4 can reach 22 ppb at an integration time of 57.8 s. Continuous measurement of atmospheric CH4 for two days validated the feasibility and robustness of our laser spectrometer, providing a promising laser spectral sensor for deploying in unmanned aerial vehicles or mobile robots.