The dielectric strength of cellulose-liquid composites is always about several times higher than that of the cellulose paper and insulating liquids. However, this experimental phenomenon has not yet been demonstrated theoretically. Herein, the spectra characterization, molecular simulation, and wave function analysis method provide a new insight that the role of nanoscale interfacial adsorption of cellulose-liquid is exclusive for composites affecting the charge separation and producing the deep-level traps to seriously hinder electromigration under an electric field, which is responsible for the difference in dielectric strength. Meanwhile, the π conjugation and σ-π hyperconjugation effects enhance the electrical stability of aromatic hydrocarbon insulating liquids. In conclusion, interfacial trap theory can be used to explain the correlation of dielectric strength between cellulose-liquid composites and cellulose paper or dielectric liquids. It can be expected that materials with high dielectric strength can be manufactured according to the fundamental study of interfacial trap theory.