Due to the features of high synchronization error and high power consumption, the classic synchronization scheme of two-way packets exchange is unfit for some applications in wireless sensor networks, especially for networks with multi-hop linear topology. This paper proposes a time synchronization protocol named Timing-sync Protocol for Linear Sensor Networks (TPLSN). The synchronization scheme of enhanced two-way packets exchange and the scheme of clock skew compensation are keys to the success of TPLSN. The phenomenon of its synchronization error accumulation over hop count is also investigated. TPLSN is evaluated on a Mica2-compatible test bed. Its synchronization error is less than 20μs for the node which is 9 hops away from the time beacon node, the increase ratio of synchronization error to hop count is less than 1μs per hop, and the increase ratio of synchronization error to resynchronization cycle is 0.017μs per second. Further, to synchronize all nodes in an n-hop linear wireless sensor network, only 2n packets are needed, which is least for any synchronization protocol based on synchronization scheme of two-way packet exchange. Theoretical analysis shows that three factors, including the approximated accuracy, the asymmetry of two-way packets exchange, and the clock skew, have great influences on time offset between two adjacent nodes. Furthermore, the clock frequency order of the linear network is found to be vital to the accumulation of synchronization error over hop count.