Wireless sensor network (WSN), which is integrated with many technologies such as sensor, embedded computing, wireless communication and distributed information processing etc., is becoming an important branch of ubiquitous computing research. WSN is composed of a large number of distributed sensor nodes, so to ensure the sensor nodes dependable is essential to security of whole network. By introducing a lightweight tamper-resistance design based on security co-processor into sensor nodes, this paper loosens up the security assumptions of WSN, and redefines the objectives of WSN security. Based on such secure infrastructure, the paper also proposes a concise and effective key pre-distribution scheme, in which each node needs to send only one cluster key announcement message and cluster key net can be set up within neighborhood.