TiO2 nanowire memristors were fabricated by dielectrophoresis. The responding current of the memristor continuously increases and decreases with the consecutive positive and negative sweep voltage, which is similar to the nonlinear transmission characteristics of biological synapses. Spike-rate-dependent plasticity and learning behaviors of TiO2 memristor were studied by applying programmed pulses. The pulses with higher amplitude, bigger width and smaller interval cause a larger excitatory postsynaptic current. The number of relearning pulses is decreased with the learning experience, and a deepening memory will be consolidated by the repeated learning process. A mechanism based on the oxygen vacancy migration is proposed for the learning behavior. Excess oxygen vacancies are generated during the learning process and the conducting pathways are formed by the vacancy drift under the applied voltage. After removing the voltage at the forgetting process, back diffusion and electron trapping of the oxygen vacancies dominate the relaxation time, and the metastable atoms are formed with the involvement of the oxygen atoms. However, weak chemical bonding among the metastable atoms leads to the migration of the regenerated oxygen vacancies again, contributing to the enhanced current in the relearning process.