Tunable diode laser absorption spectroscopy (TDLAS), as an accurate detection method for the trace gases, has been widely used in the life and production. It can accurately inverse the gas concentration through the linear relation between the integral absorbance and the gas concentration. The changes in the environment and the system noise tend to cause the deformation of the absorbance curve. Therefore, it is necessary to do the nonlinear curve fitting on the absorbance curve to make it return to the Voigt model. A real-time online monitoring system for carbon monoxide based on TDLAS has been designed and built. With the platform, a fast calculation method with the triangular Voigt linetype fitting for the single spectral integral absorbance has been put forward and compared with the Gauss-Hermite method. The accuracy of the proposed method is only 0.11% lower, but the average computation time is shortened by 84.19%. The experimental results show that the triangular Voigt linetype fitting method can greatly improve the fitting speed only with the minimal accuracy loss.