In this letter, we investigate the throughput region of wireless energy harvesting (WEH) decode-and-forward (DF) relay system with arbitrarily random data arrivals and QoS requirement characterized by the queueing bound violation probability. We derive the closed-form expression of the effective bandwidth of arbitrarily random data arrivals. In addition, the explicit expressions of outage probability and effective capacity, respectively, are acquired. Correspondingly, we derive the QoS guaranteed throughput region by combining the concepts of effective bandwidth and effective capacity. Specifically, we point out that both the first- and the second-order statistics of the random data arrivals should be incorporated into the QoS guaranteed throughput region, while the first-order statistic is sufficient to characterize the throughput region when the system can tolerate an arbitrarily long queueing delay. Finally, simulation results corroborate the theoretical analysis.