Janus nanoparticles (JPs), which are anisotropic nanoparticles with multiple constituting parts, have been recognized as superior compatibilizers for polymer-blend-based nanocomposites. However, so far, most studies focused on the effects of symmetric JPs on the phase separation dynamics of polymer blends, while the roles of asymmetric JPs during phase separation remain unclear. In this work, the phase separation dynamics of symmetric blends compatibilized by JPs with various compositions was studied by using dissipative particle dynamics (DPD) simulations. It was found that the blends compatibilized by asymmetric JPs tend to undergo morphological transitions from bicontinuous networks to droplets-in-matrix structures at the late stage of phase separation, which is due to the influence of asymmetric JPs on the energetically favored curvature of the interfaces between polymer domains. Such a mechanism is absent for symmetric JPs and other compatibilizers (e.g., triblock copolymers and homogeneous particles) because they lack the unique combination of chemical asymmetry with the particulate nature like the asymmetric JPs. Moreover, it was observed that the asymmetric JPs can stably localize at the interfaces and act as efficient compatibilizers only when the fraction of the minor constituent part exceeds a critical value. These findings not only shed light upon the roles of asymmetric JPs as compatibilizers but also indicate a promising strategy for designing polymer-blend-based nanocomposites with tailor-made structures.