This article presents a novel wavelength modulation spectroscopy gas concentration detection approach based on linear convolution (LC-WMS). The linear convolution (LC) is first time utilized in the wavelength modulation spectroscopy (WMS) measurement system to demodulate the absorption spectrum and then to obtain the absolute harmonic signals related to the gas concentration. Theoretical and experimental results show that when the absorption signal is convolved with a whole-cycle reference signal, the harmonic signals can be acquired without the need of lock-in amplifier (LIA) and low-pass filter. The LC calculation process is regarded as a combination of demodulation and low-pass filter as in the conventional demodulation system (e.g., LIA). Simulations demonstrate that appropriately increasing the cycles of reference signal (not exceeding five cycles) can help to decrease the noise of the harmonic signals. Experimental results show that the proposed LC-WMS approach can obtain a comparable accuracy as the conventional LIA in CO2 and H2O concentration measurement with concentration errors less than 5 ppm and 0.05‰, respectively. The long-term measurement shows that this method can be used for real-time detection with large concentration fluctuations. Since the LC process gets rid of the problem caused by the magnitude gain response in low-pass filter, the proposed LC-WMS method can not only obtain the absolute harmonic signal without calibration but also simplify the detection process. Thus, it has potential applications in industrial field measurements.