Free-riding destroy the foundation of BitTorrent file sharing, and result in bad system performance. The choking scheme adopted in BitTorrent nowadays can suppress the free-riding, however the coexisting unchoking scheme in which the random peers are chosen lend opportunities to free-riders. An unchoking scheme based on probabilistic link exchange, or PLX for short, is provided in this paper. The new scheme can suppress the free-riding effectively while guaranteeing the unchoking function. Free-riders can't enter into the system because they will not be unchoked by PLX after the links to free-riders are choked as PLX works based on link exchanges. Furthermore, by virtue of the mathematical designs for the probability of the link exchange, PLX can distinguish the contribution of peers, adjust their location in network according to contribution, and improve the fairness of the system. The in-depth theoretical analyses and experimental evaluations show that comparing with other methods for fighting against free-riding attacks, the PLX unchoking scheme is simple, direct and effective..