The invention discloses a preparation method of a bismuth titanate-titanium oxide heterojunction nano-material and a method for preparing a membrane electrode by utilizing the bismuth titanate-titanium oxide heterojunction nano-material so as to perform photoelectrocatalytic treatment on bacteria in ballast water. By adopting the process disclosed by the invention to prepare the bismuth titanate-titanium oxide heterojunction nano-material, the synthesis temperature is low and the preparation process is simple; by applying the bismuth titanate-titanium oxide heterojunction nano-material to prepare the membrane electrode, the preparation method is simple, the membrane electrode has relatively good stability, and the electrode can be reused; by utilizing a photoelectrocatalytic technology to apply the membrane electrode in the field of sterilization of the ballast water, equipment is simple, the sterilization time is short, the efficiency is high, the required externally applied voltage is small, and the application is favorable for ballast operation of a ship; furthermore, the sterilization treatment energy consumption is low, the process is simple, the scale is controllable, and the secondary pollution is avoided.