Gas concentration detection has always been a very important task. It is closely related to human life, environmental changes and industrial production, especially the detection of toxic and harmful gases. With the development of spectroscopy, tunable diode laser absorption spectroscopy (TDLAS) has been widely used in industrial production, environmental monitoring, meteorological sounding, etc. TDLAS has the advantages of high sensitivity, fast response, real-time monitoring and excellent portability which makes it become one of the important technologies for gas detection. It is well known that any sensor needs to be calibrated. The result of the calibration is related to the measurement accuracy of the sensor. In this paper, the direct spectral calibration algorithm based on TDLAS technology is studied. The relationship between gas concentration and transmittance logarithm in a large range is deduced and simplified. For fitting calibration 1/c fitting calibration algorithm using the relationship between gas concentration reciprocal and transmittance logarithm reciprocal is proposed. The relationship between gas concentration and transmittance logarithm is reversed by fitting the calibration curve. In the experiment, TLDAS water vapor detection platform was built. The water vapor concentration of the Vaisala HMT377 online humidity detector was used as the standard value. The water vapor concentration of 0.7%~50% was calibrated. The direct fitting algorithm and the 1/c fitting algorithm were compared. The experimental results show that the direct fitting algorithm is used to fit gas concentration to transmittance logarithm curve. The correlation coefficients of the primary function, quadratic function and quintic function are 0.946 8, 0.996 7 and 0.999 9, and the root means square error is 0.031 2, 0.007 8 and 0.001 6, the maximum relative error exceeds 100%. While utilizing the 1/c fitting calibration algorithm, the fitting curve of the relationship between gas concentration reciprocal and transmittance logarithm reciprocal is a linear function, which is consistent with the theory. The correlation coefficient is 0.999 6, and the root means square error is 0.490 1. After inverting the fitting curve, the fitting curve of gas concentration and transmittance logarithm is obtained. The correlation coefficient was 0.999 9, the root mean square error was 0.001 5, and the maximum relative error in the whole concentration range was less than 4%. The results show that the 1/c fitting algorithm is very effective. It improves measurement accuracy and expands the calibration range.