This paper proposes a new Random Channel Selection scheme with probabilistic backoff, called RCS, for a class of multi-channel MAC protocols in heavy loads WSNs to tackle the channel conflict problem. By adopting RCS, a node can reduce the probability of selecting a busy channel for data communication. Therefore, RCS can avoid data packet collision, and thus conserve more energy to extend the lifetime of WSNs. More importantly, RCS is fully distributed with no requirements of time synchronization or multi-radio, so it is practical to realize RCS in resource-constrained sensor nodes. In theoretical analysis, the probability of a channel conflict creation and the average number of misunderstood channels are obtained, which can guide the configurations of RCS. More importantly, RCS is evaluated in both simulation and testbed experiments, and results indicate that as the number of channels and loads increase, RCS significantly improves throughput and energy efficiency as well.