To evaluate the effect of liquid styrene-butadiene rubber (LSBR) on the rheological performance of bitumen, LSBRs with molecular weights of 20,000 (LA) and 50,000 g/mol (LB) are blended into the virgin bitumen. The rheological effect regarding LSBR content and the molecular weight of the bitumen was then conducted by dynamic shear rheometer (DSR) test and a bending beam rheometer (BBR) test, respectively. Short-term and long-term aging tests were used to simulate the asphalt aging with modification of LSBR, and the variations of asphalts function groups were detected by Fourier Transform Infrared spectroscopy (FT-IR). Results indicate that LSBR reduces the rutting and ageing resistance of the virgin bitumen. However, the blended LA and LB extensively enhance the crack resistance at low temperatures. Moreover, the LB modifier improves the fatigue performance of the asphalt more evidently, followed by LA, particularly at 2% mixing content (by weight of the virgin). Compared with LA, the added LB causes more deteriorating impact bitumen's rutting resistance, whereas LB increases the crack resistance at low temperatures. Of note, the LB-modified bitumen exhibits less ageing sensitivity than the LA-modified bitumen under a short-term ageing simulation. However, the LA-modified bitumen is less sensitive to the long-term ageing test.