WSNs usually operate in duty-cycle mode for long life time monitoring. This operating mode makes the communication links in dynamic, which will bring a new congestion in the network—source data congestion (SDC). Source data congestion can lead to a WSNs node to get its buffer overflowed, cause data lost and even make no response to any forwarding requirements. This problem will get worse in the sensor heterogeneous WSNs for some nodes may generate data in a burst mode. Many solutions about network congestion only focus on making data forwarding to bypass the congested node, or controlling the traffic rate. These solutions have no help on the source data congestion because this congestion is caused by the improper duty-cycle mode. According to this situation, this paper analyses the fators that influence the source data congestion, and proposes a model, called conveyor belt model, to describe the probability of source data congestion accurately. Besides, based on this model, we propose a dormant schedule method, aiming at decreasing the probability of source data congestion by reconfiguring the sleeping timer of some nodes. Furthermore, our theoretical analysis and extensive simulation show that the model can exactly predict the source data congestion, and our method can decrease the probability of source data congestion significantly.