Based on parallel-aligned liquid crystal (LC) cell, which is the prototype of LC spatial light modulator, Erickson-Leslie equation is solved, and the response formula with fixed phase retardation 2π is obtained. By theoretical calculations, the curves of relationship between cell gap and response time under various voltages are plot, the existence of optimal cell gap is ensured and the curves demonstrate that both optimal cell gap and response time decrease while applied voltage increases, and the variation is decreasing gradually; the variation of response time reaches to the maximal value: shortened by 10.62% when applied voltage changes from 5 V to 10 V. The relation curves between voltage and response time of three LC cells are measured by experiments. The results demonstrate that response time is shortened while the applied voltage increases, the variation of response time is close to theoretical value when applied voltage increases from 5 V to 10 V, and when 15 V or higher voltage is applied, response time changes little. Besides, slight thicker LC cell has better response than thinner LC cell regarding with the same variation to optimal cell gap under the same voltage.