The microscopic scale of the condensation process of water molecules from gas to liquid is extremely small and difficult to observe, which constitutes a bottleneck in the progress toward the quantization of humidity metrology. As an important method for studying the dynamic evolution of interfacial water molecules, sum-frequency generation (SFG) spectroscopy serves as a key support for promoting the transition of humidity metrology from classical macroscopic thermodynamics to a quantum-based framework. In response to the complexity of the SFG experimental platform, the wide parameter space, and the strong coupling among various parameters, a gain-factor-based intensity analysis method is proposed to optimize SFG experimental conditions and obtain higher signal-to-noise ratios. Optical parameters in the traditional SFG intensity calculation model are extracted, and a signal gain model is established to analyze the effects of angle, polarization, and wavelength. The model is applied to simulate the influence of the incident angles of infrared and visible light, the central wavelength, and polarization combinations on the SFG signal gain at the air/water interface. It is found that a synergistic response exists between the incident angles and polarization combinations of infrared and visible light, requiring joint optimization, while the effect of wavelength is mainly related to the resonance response of the material's nonlinear susceptibility. Matching the infrared frequency with the molecular vibrational frequency is identified as the key to improving output efficiency. This model provides a theoretical basis for the selection of laser parameters, optical path design, and signal enhancement strategies in SFG experiments, and holds significant value for the high-precision analysis of water phase transition dynamics and the quantum-oriented development of humidity metrology.