Coordinated multipoint (CoMP) is corroborated to be an effective technology in mitigating cochannel interference (CCI) and enhancing system performance in picocell systems, which consist of a large number of pico base stations (BSs). In picocell systems, effective CoMP clustering schemes could provide significant gains of system performance such as throughput and cell-edge spectrum efficiency (SE). Moreover, an intrinsic problem of densely deployed networks is the cost of signaling overhead and data exchange between BSs in clusters. In this paper, a novel semidynamic clustering scheme based on affinity propagation for CoMP-Pico is presented to maximize user SE and throughput under the constraint of backhaul cost. Our proposed scheme consists of online and offline stages that can achieve good performance and low complexity. Simulation results show that the proposed scheme yields significant gains of SE and throughput and low running time compared with the existing clustering schemes.