Water-in-oil emulsion is inevitably formed during the process of gathering and transportation, and the active substances such as asphaltene and naphthenic acid in crude oil may exert important effects on the emulsion stability. To explore deeply the synergistic influence mechanism of naphthenic acid and asphaltene, trans-4-n-pentylcyclohexanecarboxylic acid (trans-4) was chosen as the representative naphthenic acid while the asphaltene extracted from Tahe heavy oil is used as the representative asphaltene. The impacts of sole trans-4, trans-4 and asphaltene, and further the addition of saline ions on the emulsion stability were explored with bottle test, microscopic observation and interfacial experiment. It is found that the emulsion formed by sole naphthenic acid is much less stable than that formed by sole asphaltene. When naphthenic acid and asphaltene coexist, the oil-water interface is mainly adsorbed by asphaltene which dominates the emulsion stability. However, once the saline ions are added, they can interact with the naphthenic acid, resulting in the adsorption of more naphthenic acid at the interface. As a result, the interfacial structure is weakened and thus the emulsion stability is deteriorated.