We have calculated the project range for ions on amorphous targets made of ternary or quarternary alloys. Our calculations are based upon Vagard's law, which establishes that the project range in multi-component system can be obtained by linear combination of those of its composing binary alloy compounds. The calculated project ranges of systems, H+→GaAlAs, InAs and InGaAs, Be+→ GalnSb and InAsSb, and B+ →Cd0.2Hg0.8Te, agree well with the experimental or calculated results published in literatures. The dependences of project range on implanting energy and composition in the processes H+→ GaInAsP and GaAlAsSb, and ions→ PbSnTe are also calculated.