At present, there are few studies focusing on the preparation process of biomass oil warm mix asphalt. A rational preparation process is crucial to ensuring the performance of biomass oil warm mix asphalt. In this study, warm mix asphalt is prepared using biomass oil along with 70 #, 90 # base asphalt. The optimal preparation process of biomass oil warm mix asphalt is determined using the entropy weight Topsis method. The high and low-temperature performance of the biomass oil warm mix asphalt is evaluated by dynamic shear rheological test and bending beam rheological test. The warm mixing mechanism of biomass oil warm-mixed asphalt is studied using infrared spectroscopy and asphalt four-component test. The results show that the optimum preparation process for biomass oil warm-mixed 90 # and 70 # base asphalt involves a shear temperature of 130 °C, shear rate of 1500 r / min, shear time of 10 min and development time of 15 min. The addition of biomass oil reduces the high temperature performance of asphalt, but improves its low temperature performance. The mixing process of biomass oil and asphalt mainly involves physical blending, improving the dispersion of asphalt micelles. This results in the migration of asphalt components, ultimately enhancing the asphalt performance.