In order to reveal the relationship between the molecular composition of oil fractions and interfacial properties，emulsifying properties and clarify the mechanisms of oil-water interfacial properties change and crude oil emulsification，Bohai heavy oil was separated into asphaltene，resin and remaining fraction according to polarity difference，mass percents of three fractions being of 5.45%，26.5% and 57.13%，respectively. Molecular composition of three fractions was investigated by infrared spectrum，element analysis and high-resolution mass spectrometry. Moreover，interfacial tensiometer and interfacial viscoelastic instrument were used to study the interfacial properties. The results demonstrated that the asphaltene and resin was rich in acidic compounds. Moreover，the relative molecular weight，the degree of condensation and the content of heteroatom of acidic compounds in asphaltene were higher than that of resin. The interfacial tensions between model formation water and model oil of 0.55% asphaltene，5% resin，5% remaining oil，was 11.5 mN/m，20 mN/m，28 mN/m，respectively，which indicated that the acidic compounds in crude oil were the main interfacial active materials，which with heteroatoms exhibited stronger interfacial activity. The interfacial shearing viscosity of three fractions model oils decreased with increasing shear rate，meantime，at shear range of 0.02—0.4 rad·s^-1，three fractions was arranged according to the interfacial shearing viscosity as follows：asphaltene>resin > remaining fraction，which indicated that the interfacial film formed had obvious structure，especially for asphaltene. The W/O emulsion formed by crude oil and ethanolamine solution presented high stability for more than a week. The stability decreased obviously by removing the asphaltene in crude oil，and the oil-water separated completely after 11 h；while，the emulsification did not appear as both of asphaltene and resin were removed，which suggested that the asphaltene and resin were the main active components for emulsification of Bohai heavy oil.