The　formation　mechanism　of　the　amorphous　Ni-Fe-P　coating　was　studied　by　analysis　of　the　forming　thermodynamics,　dynamics,　and　crystallography　of　the　amorphous　alloy.　The　results　show　that,　in　the　initial　stage　of　deposition　a　thin　crystal　epitaxial　growth　layer　first　forms,　and　then　transforms　to　amorphous　gradually.　The　cross　section　in　Ni-Fe-P　coatings　by　electrolytic　etching　exhibits　a　banded　structure　of　alternate　dark　and　light　bands.　It　is　proposed　that　the　banded　structure　is　caused　by　a　change　in　the　P　content　with　thickness,　which　is　due　to　alternated　depletion　and　enrichment　of　[OH-]　in　the　diffusion　layer　resulting　from　the　generation　and　evolution　of　hydrogen　gas.　The　amorphous　Ni-Fe-P　coating　will　be　formed　in　proper　composition,　high　nucleation　rate　and　strongly　hindered　growth　of　the　crystal　nucleus.　Amorphous　Ni-Fe-P　alloys　form　as　islands,　and　grow　up　by　layer.