More attention has been paid to congestion control in the emerging area of wireless sensor network (WSN). How- ever, most research works in the past stayed at the level of the current algorithms design or modification, and sel- dom sought solutions on the viewpoint of architecture. In this paper, Backpressure(BP) under Active Network(AN) ar- chitecture is used to make congestion control more respon- sive to detect/recover congestion in WSN. We design a sim- ple Active Backpressure mechanism to allocate bandwidth Proportional to the Size of tree (ABPS), and we present a fluid-based analytical model of ABPS using stochastic differential equations. ABPS introduces programs in each data packet that tell nodes how to react to congestion, and quickly converge to a fair and efficient rate. We demonstrate a deterministic approach to analyse the stochastic model, in which we obtain a set of ordinary differential equations from our model, and we derive the average behavior of queue length and flow throughput from the ordinary differ- ential equations. Finally, we evaluate ABPS extensively on a 50-node wireless sensor network. Simulation results vali- date the effectiveness of our ABPS and match well with the theoretic analysis.