Key distribution plays an important role in wireless sensor networks (WSNs), for information must be kept secure, even in an environment with limited storage ability and low data processing speed. However, it is a challenge to work in such a harsh requirement environment, the traditional key management scheme such as key distribution center (KDC) and public key cannot be used. Key pre-distribution is a good way to address this problem in WSNs, and while several schemes have been presented before, they cannot perform well in a large scale network. In this paper, we first use node's deployment knowledge to propose a hexagon scheme, and then combine it with the bivariate-polynomial to realize a new key agreement scheme. The simulation results show that our new scheme can be used in a large scale network, is attack resistant with low memory overhead, and achieves good network connectivity. We also introduce the measure how to effectively use key pre-distribution skills in the routing protocols.