An electrostatic surface well scheme for trapping cold polar molecules in week-field-seeking state is proposed, and the electrostatic surface well is composed of a pair of metallic parallel-plate capacitors and four charged rods part-embedded in insulating medium. Spatial distribution of electric field is calculated with the finite element software in the trapping process in the case of single well trapping, and we find that a three-dimensional enclosed electrostatic well which is 2.2 mm above the chip surface is obtained. Ammonia molecule (ND3) is chosen as the test molecule and the dynamic processes of loading and trapping are simulated by the classical Monte Carlo method. Simulation results indicate that when the velocity of ND3 molecular beam center is 13 m/s and the loading time is 0.576 ms, the loading efficiency as high as 53% can be obtained, and the corresponding temperature of trapping cold molecules is about 35 mK. When the voltage applied to the flat electrode is increasing, the single well will splits into two symmetrical wells, and the ratio of the molecules in the two wells is 1∶1. Unsymmetrical split of the single well can be realized when we adjust the voltages of the two pole electrodes in the middle, and the ratio of trapping molecules in the two wells can be controlled. The ratio of molecules in the left well or the right well to the total molecules can be adjusted from 0% to 100%. This method lays a foundation for the three-dimensional trapping cold polar molecule electrostatic surface interferometer, and it is helpful for the studies about precision measurement and matter wave interference.