As is well-known, when electrostatic equilibrium is reached, the electrostatic field outside a metal is always perpendicular to the metal surface and there is no component parallel to the metal surface. However, unexpectedly, in this work the electrostatic field parallel to metal surface, i.e., the tangential electrostatic field (TEF) at metal surface, may be discovered by means of mechanical-electric coupling which may originate from the strain-dependent Fermi surface energy (FSE). The TEF at metal surface and the related electrostatic field inside the metal may not only give birth to new knowledge on electrostatics of metals but also induce interesting physical effects. For the electrostatics of metals, new understandings of electrostatic equilibrium conditions and related boundary conditions, Uniqueness theorem, method of image charges, electrostatic shielding, Thompson’s theorem and Green’s reciprocation theorem were gained and thereby some more accurate modifications were made. For the related physical effects, some new interpretations on the photoelectron emission spectroscopy, redistribution of minority atoms at surface of nanoparticles and TEF-induced Reshba effect were proposed. Moreover, interestingly, it was revealed that an intrinsically intensive electrostatic field may accompany the nanoparticles, and this field may cause an electron double-dipole resonance (EDDR) mechanism in vicinity of nanoparticles. Using EDDR, a simply unified model might be constructed for the longstanding problem in physics and chemistry, i.e, surface-enhanced Raman scattering (SERS). And this unified model could provide satisfactory explanations for the main experimental phenomena on SERS. In a word, the unraveled TEF at metal surface and the related electrostatic field inside a metal may enable people to re-understand the electrostatics of metals and explore subsequently mechanical, physical and chemical effects in various areas.