The rapid development of network technology and the advent of various smart devices can cause the rapid growth of network data traffic in the hot areas. However, the existing cellular network capacity cannot meet the needs of network users. Therefore, the small cellular base stations (SCBSs) are proposed and widely used. Nevertheless, the physical cell identifications (PCI) are extremely limited, thus it is difficult to reasonably allocate the limited PCIs to the massive SCBSs, which greatly affect the quality of service (QoS) for users in hot areas. In this paper, a PCI allocation and reuse scheme based on the maximum degree first coloring (MDFC) algorithm is proposed to allocate the limited PCIs for SCBSs in hot areas. Moreover, the K-means clustering algorithm is adopted to divide and identify different hot areas with the mobility data of massive users, which also contributes to the allocation of PCIs. The proposed scheme reduces the rate of PCI conflict and confusion, and improves the users’ QoS. Simulation results show that the MDFC algorithm has better PCI allocation efficiency, and shorter PCI allocation time. Thus, it is more suitable for the deployment of cellular networks in hot areas.