Lithium ion transport in a polycrystalline solid-state electrolyte (SSE) is directly linked to the properties of lithium ion batteries. Grain boundaries (GBs), as essential defects in SSE, have been found to play a significant role in the overall kinetics of lithium ion transport, however, the mechanism is not well understood due to the complex role of GBs. The GBs could affect the overall kinetics of ionic transport in the SSEs in two ways: (i) Li/Na diffusivities inside the GBs could be different from those in the bulk, and (ii) point defect segregation at the GBs. The first aspect is well recognized, whereas the second one has been rarely studied. In this study, a combination of first principles and phase field calculations were performed, in which the interaction between point defects and grain boundaries were considered at different scales, to reveal the role of GBs in the overall ionic conduction of SSE anti-perovskite Li₃ClO. The results show that defect segregation, which varies significantly with the GB orientation, reinforces the negative contribution of GBs on the overall ionic diffusivity by approximately one-order of magnitude. This study could help improve the fundamental understanding of ionic transport in polycrystalline SSEs, and provide guidance for the design of new SSEs with excellent ionic conductivity.