A complex “frequency-doubling-comparison-phase-shifting” circuit is often required for the generation of frequency-doubling signals for harmonic detection in the existing tunable diode laser absorption spectroscopy-wavelength modulation spectroscopy (TDLAS-WMS). Thus, an innovative signal generation method is proposed in this paper. In this method, a field programmable gate array (FPGA), as a main controller, controls a high-speed digital-to-analog (D/A) converter to generate a sawtooth scanning signal and sine modulation signal, which are used to drive a laser for wavelength modulation scanning after the signals are combined. Simultaneously, a square signal, which is frequency-multiplying and strictly in-phase with the sine wave, is also generated based on the phase information for demodulation of the gas absorption signal. Based on this method, the system scheme, high-speed D/A conversion circuit, and FPGA control flow are designed, and the generation of the sinusoidal signal and its frequency-doubled square wave signal with a delay is verified experimentally. When the system is applied to a gas detection, an absorption signal related to the concentration is successfully detected, and the detection amplitude is improved to some degree, which verifies the effectiveness of the proposed method.