•AO/NO reduced the viscosity of CRMA.•The pre-swelling process had great influence on the properties of CRMA-AO/NO.•The influence of pre-swelling process on CRMA was related to oil content.•The temperature variation during the preparation of CRMA was simulated by MD simulation.•The macro properties and MD simulation were used to reveal the viscosity reduction mechanism of CRMA-AO/NO. Using light oil to pretreat the crumb rubber (CR) could reduce the viscosity of the modified asphalt, thereby enhancing its compatibility with asphalt binder and improving the workability of CR modified asphalt (CRMA). However, the effects of CR pre-swelling on rheological properties of CRMA and the viscosity reduction mechanism are not well understood, especially for commonly used aromatic and naphthenic oils. In this study, the comparative investigations on rheological properties of CRMA before and after pre-swelling process were performed by frequency sweep, temperature sweep, multiple stress creep and recovery (MSCR) and storage stability tests. Meanwhile, the molecular dynamic (MD) simulation method was applied to simulate the distribution, migration, and aggregation characteristics of aromatic oil, naphthenic oil, and asphalt components in CRMA from natural aggregation state to heating mixing and finally cooling, aiming to reveal the viscosity reduction mechanism. The results showed that aromatic and naphthenic oils can significantly reduce the viscosity of CRMA. In addition, the low-temperature crack resistance and storage stability of CRMA can be improved through pretreatment with aromatic and naphthenic oils, which is correlated with the oil content. However, the effects of aromatic and naphthenic oils pre-swelling processes on the high temperature properties of CRMA are different. The MD simulation results showed that the binding energy between asphalt molecules reduced after mixing with aromatic and naphthenic oils, which is which is the primary reason for the viscosity reduction of CRMA. However, there are differences regarding the distribution characteristics of these two oils within asphalt binder. The CR, aromatics, aromatic oil and resins were uniformly dispersed in CRMA system by aromatic oil, whereas the naphthenic oil was adsorbed onto the surface of CR and asphaltenes.