With the advances in information and communication technology, the unstructured mobile peer-to-peer (MP2P) system based on a hybrid mobile ad-hoc network (hybrid MANETs), where the membership between mobile nodes is more dynamic and the energy on each node is limited, becomes a trend. Since different mobile nodes may store copies of shared information, data synchronization on an MP2P system is crucial for providing the latest information. Nevertheless, the node mobility makes the communication links disconnected and the network topology changed frequently. The maintenance on data consistency in an MP2P system thus becomes challenging. In this paper, we propose a heuristic data update mechanism (HDUM) to effectively maintain the data consistency in an MP2P system with less overhead. In HDUM, each node derives safe-time values from the neighbors’ locations and speeds to determine an appropriate time to check and update data items between the neighboring nodes and itself. HDUM also considers the relative node ability, which is derived from nodes connectivity, access frequency of a data item, and the residual capacity of battery. Relative node ability indicates the ability of a node with respect to a data item. If the ability value of a node is not lower than a given threshold, this node can be notified to receive updated data items from its neighbors. To evaluate the proposed HDUM, extensive simulated experiments are performed. The experimental results show that HDUM can keep higher coverage of successfully synchronized nodes while significantly reducing a large amount of redundant messages.