Abstract: Experimental studies on the spontaneous nucleon emission have enabled the exploration of new isotopes beyond the drip line, as well as revealing their unique structural and decay characteristics. Furthermore, such studies are of critical importance for exploring the limits of nuclear stability and understanding the nucleon-nucleon interactions under extreme conditions of isospin asymmetry. This paper introduces our experimental study on the four-proton decay of ^18\rmMg, a new magnesium isotope beyond the proton drip line. Firstly the radioactive nuclear beamline and detector appratus are introduced, and the results of the particle identification of beam particles and decay fragments are presented. Then the scintillating fiber array is mainly introduced together with its data analysis method. With its performance of detection efficiency and position resolution, this array has been proven by simulation to be very necessary for improving the energy resolution of the invariant mass spectrum. Finally, with the five-body coincidence measurement of ^14\rmO+4\rmp, the decay energy spectrum of ^18\rmMg was constructed using the invariant mass method. The experimental results can be relatively well described by the Gamow Shell Model, demonstrating the significant impact of continuum coupling effect on the structure of nuclei beyond the drip line. The measured 2^+_1 excitation energy in ^18\rmMg is higher than that in the traditional magic nucleus ^20\rmMg, which suggests that a possible demise of the N = 8 shell closure or some exotic nuclear structure effect may occur in this extremely proton-rich Mg isotope.