In cloud platforms, resource provision service plays an important role to provide flexible and reliable IT-infrastructure for various kinds of applications. Unfortunately, current resource provision service seldom support resource reservation which is generally applied for improving the quality of service QoS in high-performance systems. To address this problem, a virtual machine provision policy based on elastic reservation mechanism is proposed. It extends the conventional 'on-demand provision' by introducing a novel elastic reservation mechanism, which allows cloud providers to accept user's over-reservation requests under certain conditions so as to improve the resource effective utilisation and user's satisfaction. Theoretical analysis presents the approach to calculating the probability of elastic reservation, and experimental evaluation validates the effectiveness of the proposed policy and the effects of key parameter on system performance. The results show that comparing with existing techniques, elastic reservation-based resource provision policy can significantly increase the resource effective utilisation, and decrease the 'short-term resource under-provision' caused by excessively reservations.