Freshness is one of the most desired security properties of sensor network communication architecture. And sequence number is a common technique that is used as a freshness identifier to achieve freshness transmission. However, due to characteristics of wireless sensor network, such as resource-constrained and attack prone, it is vulnerable and energy-expensive to transmit sequence number directly. In this paper, we propose an Energy-efficient Freshness Transmission Protocol (EFTP) which lowers overhead as well as improves the security of sensor network. In EFTP, each sequence number is disarranged via a tree-architecture and further divided into two correlative parts. Encrypted and attached to messages, sequence number can be transferred securely. The receiver reverses process of above operations to authenticate the freshness of received message. Compared with previous works, EFTP achieves the freshness requirement without any redundant overhead or keeping synchronized sequence number. Simulations show that our solution performs very well not only in freshness requirement but also in energy-efficient.