In this paper, we present a new contention access method for scalable wireless sensor networks. In these networks, the traffic levels are often variants, particularly in monitoring applications. Sensors do not regularly have much data to send. However, when an event occurs, every sensor in a given area will send its alert to the access point simultaneously. Hence, this network suffers a huge number of communications at the same time. In fact, at an instant, only one transmission is permitted among interfering nodes. If there are more than two transmissions at the same time, there will be a collision. Therefore, this variant communication of wireless sensor networks leads to an increase in collision and energy consumption. Traditional wireless network uses an exponential back-off scheme to react to collision. However, this scheme is designed for traditional wireless networks and is not well adapted to wireless sensor networks. Our proposal uses a dynamical change of channel access probability which can reduce the number of contention times and collisions. Moreover, it can be applied in any existing MAC protocol of wireless sensor network.