In order to reduce the viscosity and improve the fluidity of viscous crude oil， the effect of the dosage of viscosity reducer consisted of alpha olefin sulfonate（AOS）and sodium carbonate（Na₂CO₃）on the viscosity reduction of viscous crude oil were studied and the mechanism of emulsification and viscosity reduction was analyzed. The results showed that the temperature resistance of AOS was good. The stability of O/W emulsion increased with increasing temperature. The effect of AOS on the emulsification and viscosity reduction of viscous crude oil was good. The viscosity of viscous crude oil emulsion decreased gradually with increasing AOS dosage. The viscosity reduction effect of viscous crude oil was the best with 81.95% viscosity reduction rate when the mass fraction of AOS was 2%. AOS and Na₂CO₃ compound system had synergistic effect on emulsification and viscosity reduction of viscous crude oil. The viscosity reduction effect of viscous crude oil after mixing with 1% Na₂CO₃ and 0.05% AOS in 1∶1 volume ratio was the best， and the viscosity reduction rate of viscous crude oil was 98.22%. Compared with the use of AOS alone， the viscosity of viscous crude oil emulsion decreased with the combination of AOS and alkali， the AOS dosage reduced and the economic benefit improved.