In the convergence scenario of wireless sensor networks (WSNs) and cellular network, the mobile gateway UEs of cellular network need to provide service to larger numbers of sensor nodes, in order to reduce energy consumption and increase the network lifetime of WSNs. Nevertheless, the load balance of sensors among mobile gateways is a new- come research points in the convergence system. In this paper, we focus on the mechanisms of load balancing between gateway UEs in convergent system and propose a BS-assisted load balancing algorithm in the context of a multi-gateway wireless sensor network with cluster tree topology. Simulation results show that the proposed algorithm resolves the load unbalance problem in WSNs, especially between different personal area networks (PANs) effectively.