Microscale flyash/SBS composite modified asphalts (CMA) were prepared by melt blending. Rolling thin film oven test (RTFOT) was employed to study the anti-aging property. Both flyash and SBS disperse homogeneously in the base asphalt, which lead to an improvement in terms of toughness, strength, and thermal stability. Moreover, microscale flyash plays a significant role in the CMA. The effect of flyash on the properties of SBS modified asphalt appears as a decreasing penetration, an increasing softening point, and a decreasing ductility. The anti-deformation ability under high temperature of the CMA improved significantly. In comparison with that of the base asphalt, the softening point of the CMA decreases after aging, which maybe attribute to the different rheological properties between the CMA and the base asphalt. Compared to that of SBS modified asphalt, the anti-aging property of CMA improved significantly. This demonstrates that the microstructure of flyash can prevent oxygen from diffusing and permeating.