Electric field ionization of the Li 25P Rydberg state is studied theoretically with the three-step resonant excitation，2S1/2—2P3/2—3D3/2—25P ，and ionized by an external electric-field pulse. The rate equations for the related particle population of different atomic states are set up for the above process, from which the analytic solutions of the particle populations of different states and ionization efficiency are derived with the Laplace transformation. In order to explore the physical mechanism and characteristics of each step during the process, a home-made Matlab programs is written. The impact of both parameters of laser and electricfield pulses used in photoexcitation and electric-field ionization processes on the ionization probability has been analyzed quantitatively. The study indicates that with the present excitation scheme photoexcitation efficiency of the 25P Rydberg state is hard to be improved significantly, leading to the limitation of overall efficiency of the electric-field ionization.