The length of physical slot is stationary in the time division duplex mode of the Massive MIMO system. Consequently, the length of the allocated physical slots cannot be adjusted according to different lengths of coherence intervals. The mismatch between slot and coherence leads to a waste of the time-frequency resource. Therefore, the whole network capacity of the system is not optimal. A variable-length slot allocation scheme based on hierarchical users was proposed to make the allocated physical slots match their corresponding coherence intervals well. In the proposed scheme, terminals were divided into different categories according to their mobility, and variable length slots were assigned to terminals in different categories flexibly. So the waste of time-frequency resource waste and the excessive channel estimation can be alleviated. The resource efficiency and the sum rate of the system are verified to be optimized and the simulation validates the proposed scheme. 