Device-to-device (D2D) communications have been proposed as a promising technology to improve network capacity and user experiences. In this letter, we make theoretical analyses on D2D mode selection with user mobility. Using the stochastic geometry, we deduced that the D2D communication range can be well represented by biased circles with varying radius. Besides, a tractable analytical framework considering both absolute movement and relative movement for mode selection in moving D2D scenarios is presented. Under the proposed model, the closed-form expressions of D2D mode transition rate and D2D mode residence time are derived, which are the key parameters for moving D2D mode selection performance.