In the downlink collaborative transmission process of the power Internet of Things(IoT), it is necessary to optimize the effective capacity of the system while ensuring low-latency communication, in order to continuously provide high-quality services to users. A Quality of Service(QoS)-Guaranteed strategy is proposed based on Remote Radio Heads(RRH) Collaboration(QG-RRHC). A two-layer power IoT network model based on Orthogonal Frequency Division Multiple Access(OFDMA) is designed. By introducing the theory of effective capacity, a distributed RRH collaborative transmission scheme for QoS guarantee is studied, which collaboratively serves the downlink data transmission on different subcarriers. Additionally, a heuristic optimization algorithm based on Lagrange duality is proposed to solve the joint optimization problem. Simulation results demonstrate that compared with other benchmark algorithms, the proposed strategy can significantly enhance the system's effective capacity and achieve the performance close to the optimal.