Unmanned aerial vehicles (UAVs) as the aerial relay become a highly desired scheme to assist terrestrial network. In this work, we intend to utilize the UAV swarm to assist the communication between the base station (BS) equipped with the planar array antenna (PAA) and the IoT devices by collaborative beamforming (CB). Specifically, we formulate an average achievable rate and energy bi-objective optimization problem (AREBOP) to improve the average achievable rate of IoT terminal devices and energy consumption of UAV swarm by jointly optimize the excitation current weights of BS and UAVs, the position of UAVs and user association order of IoT terminal devices. Moreover, the formulated AREBOP is proved to be NP-hard. Thus, we proposed an multi-objective grasshopper algorithm with specific initialization (MOGOASI) to solve this problem. Simulation results show the effectiveness of MOGOASI and illustrate that the performance of MOGOASI is superior compared to some benchmarks.