A simple physical model and theory have been proposed to investigate the electrocompression behavior of electrodeposited halide (chloride, bromide, and iodide) monolayer at the Au(111) electrode surface. By establishing the statistical thermodynamic formulas, the effective pair interaction energy, 2D spreading pressure, 2D isothermal compressibility, and electrode potential-coverage (theta-E) curves of halide monolayer at Au(111) surface have been calculated. The theoretical results are in good agreement with the experimental measurements, which show that the adatom-adatom interactions, especially repulsive interactions, determine the size of the compressibility in the halide adlayer on Au(111) surface with an increase in the electrode potential. The electrosorption valency gamma of halide also plays an important role on the electrocompression. The theory in this paper is in principle applicable to other anion species at metal electrode surface.