Abstract: Proton decay of nuclei beyond the proton dripline provides a key spectroscopic tool to investigate the nuclear structures and nuclear stabilities under extreme isospin conditions. Recent experimental studies on the nuclear proton decay are first summarized, followed by the brief introduction of the major approaches of producing nuclei beyond the proton dripline, the main experimental methods of studying their proton decays, and the understanding of the decay mechanisms of dripline nuclei. On this basis, our recent experimental research on the four-proton decay of ¹⁸Mg is presented. The new isotope ¹⁸Mg, which is located two neutrons beyond the proton dripline, has been experimentally observed for the first time by using the invariant mass method. The decay energies and decay widths of the ground and first 2⁺ states in ¹⁸Mg have been determined. As the measured excitation energy of the2_1^\text + state in ¹⁸Mg exceeds that in ²⁰Mg, this observation provides an argument for the demise of the N = 8 shell closure in nuclei far from stability. The observed momentum correlations between the decay products suggest that ¹⁸Mg_g.s. mainly decays by two sequential steps of prompt 2p decay, passing through the ground state of ¹⁶Ne.