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With ultra-dense networks (UDNs) becoming one of key technologies for the fifth generation (5G) networks, it is an important problem to evaluate the impact of user mobility on UDNs. In this paper, random way point mobility models and the stochastic geometry theory are utilized to evaluate the energy efficiency of 5G UDNs. Considering the random deployment of base stations, the coverage probabilities of UDNs are derived for macro-cell users and small-cell users, respectively. Moreover, the network capacity and energy efficiency are evaluated for UDNs considering the user mobility. Simulation results indicate that the network capacity and energy efficiency does not monotonously increase with the increase of the BS density when the user mobility is considered. Compared with the existed studies, a new method is proposed to evaluate the impact of user mobility on the future 5G UDNs.