A topological analysis on the ternary 7-phase multisystem involving laihunite (FeO-Fe_2O_3-SiO_2; Q-H-M-W-L-Fa-fs) in P-T plane has resulted in an n+4 phase closed-net-diagram. The most possible straightline-net-diagram was derived by using the thermodynamic properties of the phases concerned. From the straight-line-net-diagram it is apparent that laihunite is not a phase stable only at high pressures. P-T-fo_2, analysis suggests that the formation and stabilization of laihunite are closely related to oxygen fugacity. However, laihunite shows different ways of formation at different pressures. At low pressures, it is formed via the oxidation of fayalite; with increasing pressures the contribution of ferrosilite to the formation of laihunite increases; at very high pressures laihunite can be formed only by the oxidation of ferrosilite. A topological analysis on the ternary 7-phase multisystem involving laihunite (FeO-Fe_2O_3-SiO_2; QHMWL-Fa-fs) in PT plane has resulted in an n + 4 phase closed-net-diagram. was derived by using the thermodynamic properties of the phases concerned. From the straight-line-net-diagram it is apparent that laihunite is not a phase stable only at high pressures. PT-fo_2, analysis suggests that the formation and stabilization of laihunite are closely related to oxygen fugacity. However, laihunite shows different ways of formation at different pressures. At low pressures, it is formed via the oxidation of fayalite; with increasing pressures the fossil of ferrosilite to the formation of laihunite increases; at very high pressures laihunite can be formed only by the oxidation of ferrosilite.