Sum-frequency nonlinear spectroscopy is a powerful tool in investigating physical and chemical properties at gas/liquid, gas/solid, liquid/liquid and liquid/solid interfaces. Fermi resonance is a well-documented anharmonic phenomenon related to molecular vibrational coupling and the energy transfer phenomenon that exists within and between molecules. Such a phenomenon is widely used in the fields of materials, biology and chemistry. Combining density functional theory and molecular dynamics simulation, we present a method of studying sum-frequency vibrational spectroscopy for the CH3 group of methanol at interfaces due to Fermi resonance. The calculated spectroscopic data agree with the experiment and provide a novel and untraditional point of view with respect to traditional approaches.