The use of an appropriate viscosity reducer is the key during the process of heavy crude oil exploitation. To design a new amphiphilic polymer as an efficient viscosity reducer, we combined the dissipative particle dynamics (DPD) simulation and all-atom simulation in this work to reveal the viscosity reduction mechanism. Nine primary polymers were added into the oil-water system, in which the oil-water volume ratio was 4:1, by comparing the mean-square displacement (MSD) of asphaltene, aggregation ratio, the oil-water interfacial tension value, and the effects of nitrogen type, benzene ring morphology, and the alkyl side-chain length of the polymers on the properties of the system. The interaction mechanism was further revealed by analyzing the interaction energy between the asphaltene and the polymer molecules at the atomic scale. Finally, three kinds of polymers, P12, P22, and P31, which used polyacrylamide as a skeleton and were modified by pyrrolidone, a naphthalene ring, and the alkyl side chain without a -CH2 group, respectively, were selected to design a new amphiphilic polymer as a viscosity reducer. This work combined the advantages of the mesoscopic and atomic simulation methods to design the amphiphilic polymer, shedding light on the development of the novel heavy-oil viscosity reducer.