The　crystal　structure,　voltage　and　capacity　of　Li1-xNi0.5Mn1.5O4　were　studied　by　using　the　first　principle　theory.　The　results　show　that　the　structure　of　Li1-xNi0.5Mn1.5O4　has　changed,　and　the　lattice　parameter　decreased　from　8.342　angstrom　to　8.247　angstrom　with　the　decrease　of　Li　content.　But　the　change　in　each　direction　is　the　same,　so　the　octahedral　structure　is　unchanged　in　the　process　of　de-lithium.　The　length　of　Ni-O　and　Mn-O　bonds　decreases　with　the　increase　of　de-lithium.　The　Mn-O　bond　decreases　from　2.101　angstrom　to　1.911　angstrom,　while　the　Ni-O　bond　decreases　from　2.065　angstrom　To　1.965　angstrom.　The　total　energy　decrease　from　-28731.220　eV　to　-27178.422　eV,　and　the　voltage　decreased　from　4.7　V　to　4.3　V　with　the　reduction　of　Li　amount.　All　the　formation　energy　of　Li1-xNi0.5Mn1.5O4　(P4332)　is　negative,　which　display　that　the　structure　of　crystal　is　stable　with　no　phase　separation.　The　theoretical　capacity　reduced　from　146　mAhg(-1)　to　0　with　the　increase　of　delithiated　amount　or　Li/Mn　molar　ratio.