Currently, the method that uses a first-order Taylor series to approximate laser transmission has seriously affected the gas concentration measurement accuracy of tunable diode laser-absorption spectroscopy (TDLAS). This paper employs a second-order Taylor series to approximate laser transmission, and a high-precision second-order algorithm has been established that can determine the gas concentration directly. Then, this algorithm is used to test the NH₃ mole fraction in a cell with NH₃-Air mixtures. Experimental results show that the second-order algorithm not only effectively improves the measurement accuracy of gas concentration but also greatly broadens the scope of TDLAS.