The burst contention has a strong impact on the performance of optical burst switching (OBS) network, and currently, various contention resolution schemes have been proposed from time domain, space domain and wavelength domain, respectively. Among the schemes, the deflection routing is more attractive, which can loosen the requirement of optical buffer in capability and quantity. With deflection routing, the contending burst is rerouted to an available port other than the intended output instead of being dropped. However, when the traffic load is high, there should not be only more contentions but also frequent deflection routings, some of which might aggravate the network status. To improve the status, this paper proposes the tunable-parameter deflection routing (TPDR) algorithm, which can control the deflecting operation of a contending burst by a tunable-parameter, i.e., deflection probability, which is settled by the priority and traffic load of contending burst, and search the optimum deflection path in terms of the burst loss probability and the path length. The simulation results show that it can improve the overall BLP and the BLP of each individual priority, and alleviate the negative effect of the offset-time deficit on QoS guarantee.