A hypothesis on electrical field distribution is presented for the physical model of a scanning probe tip with a conical body ending in a spherical apex. Based on the hypothesis, we deduced an analytical model to calculate the electrostatic force acting on the tip and analyzed variation of the electrostatic force with structural parameters of the tip. The simulation results are consistent with those given by numerical methods of equivalent charge distribution (ECD) and by experiments. The hypothesis is proved reasonable according to the work.