In view of the great significance of the high-voltage transmission line on-line monitoring system to protecting the transmission line safe running， a hierarchical heterogeneous wireless sensor network is designed and implemented according to the layout of the high-voltage transmission lines and the characteristics of the parameters monitored. The network consists of a backbone network and a sub-network， the latter adopting Zigbee technology being responsible for collecting images and scalar data. The backbone network is based on IEEE 802.11 multi-hop ad hoc network that enables reliable long-distance transport. A high-voltage multi-parameter monitoring system is designed and implemented. Some key problems during implementation are discussed and an application case is presented. The system designed is especially suitable for high-voltage power line through remote districts for its high reliability， scalability and low cost.