In large-scale wireless sensor networks (WSNs), when the consumption of hardware is limited, how to maximize the performance has become the research focus for improving transmission quality of service (QoS) of WSNs in recent years. This paper presents a new evaluation method for packet buffer capacity of nodes using queueing network model, whose packet buffer capacity is analyzed for each type node, when it is in the best working condition. In order to evaluate congestion situation in the queueing network, and to get real effective arrival rates and transmission rates in the model, holding nodes were added in the queueing network model, and equivalent queueing network model is expanded. We establish an M/M/1/N type queueing network model with holding nodes for WSNs and design approximate iterative algorithms. Experimental results show that the model is consistent with the real data.