Cooperative transmission can significantly reduce the power consumption associated with long distance transmission in wireless sensor networks (WSNs). In this paper, we analyze the optimal power consumption of cluster-based multi-hop transmission for a cooperative WSN. With specific Quality of Service (QoS) constraints on delay and channel capacity, we show that the power optimization problem of the whole network has no closed-form solution in a slow flat Rayleigh fading environment. Thus we propose a dynamic power control and optimization (DPCO) scheme that can jointly determine the optimal number of cooperative sensors and their transmission power. We further propose a channel approximation algorithm that can significantly reduce the computational complexity of the DPCO scheme.