Subtraction, division, and balanced ratiometric detection (BRD) are three extensively used demodulation methods for dual-beam wavelength-modulation trace gas detection. However, reliability comparisons among these methods under changing environmental conditions were rarely researched. In this paper, the influences of ambient temperature and bend loss of fibers are analyzed in detail, and the reliabilities of the subtraction, division, and BRD methods are quantitatively compared for the first time to our knowledge. When the ambient temperature is increased by 1°C, the deviation of the division method is only 0.29%, which obviously outperforms the subtraction method (2.90%) and the BRD method (0.55%). Furthermore, a concept, "power fluctuation rejection ratio," is introduced to compare the suppression effects of the subtraction, division, and BRD methods on the laser light source power fluctuation. The study results demonstrate that the division method provides better reliability when the ambient temperature or bending loss is varied. The validity and reliability are fully verified by the fact that the experimental results give good agreement with the theoretical simulation.