Asphalt deposition is a key challenge faced in crude oil extraction and transportation，which not only leads to a decrease in oil well production，but also causes the pore channels blockage during oil extraction and pipeline blockage during transportation in severe cases. The most common strategy to prevent or reduce asphaltene deposition is to use asphaltene dispersants to increase asphaltene stability and reduce asphaltene aggregation. The purpose of this paper was to elucidate the effect and mechanism of asphaltene dispersant on asphaltene stability. 4-Nonylphenol（NP）was selected as the asphaltene dispersant，and the effect of dispersant on asphaltene stability was investigated by the mass-fraction conductivity method. The mechanism of the dispersant was investigated from the perspective of tertiary structures，such as the molecular structure，the structure of aggregates and the structure of clusters，respectively，by means of FT-IR，XPS，XRD，TEM，particle size distribution，and zeta potential analysis，etc. The results showed that the addition of dispersant significantly improved the stability of asphaltene，which was adsorbed on the surface of asphaltene molecules through intermolecular hydrogen bonding and other interactions，and increased the spatial resistance between asphaltene molecules，increasing the asphaltene lamellar spacing from 0.34 nm to 0.36 nm，which also reduced the asphaltene binding，resulting in the disassembly and depolymerisation of asphaltene binders and their cluster structures based on the original behaviour. The number of layers of asphaltene aggregates decreased from 7.15 to 5.49，the particle size of the asphaltene clusters rapidly decreased by more than 70% and the zeta potential increased significantly. Finally，the asphaltene clusters were prevented from further growth into flocs，thus reducing and preventing asphaltene deposition.