The　structures　and　elasticities　of　phase　B　silicates　with　different　water　and　iron　(Fe)　content　are　obtained　by　first-principles　simulation　to　understand　the　effects　of　water　and　Fe　on　their　properties　under　high　pressure.　The　lattice　constants　a　and　b　decrease　with　increasing　water　content.　On　the　contrary,　c　increases　with　increasing　water　content.　On　the　other　hand,　the　b　and　c　decrease　with　increasing　Fe　content　while　a　increases　with　increasing　Fe　content.　The　decrease　of　M　(metal)-O　octahedral　volume　is　greater　than　the　decrease　of　SiO　polyhedral　volume　over　the　same　pressure　range.　The　density,　bulk　modulus　and　shear　modulus　of　phase　B　increase　with　increasing　Fe　content　and　decrease　with　increasing　water　content.　The　compressional　wave　velocity　(V-p)　and　shear　wave　velocity　(V-s)　of　phase　B　decrease　with　increasing　water　and　Fe　content.　The　comparisons　of　density　and　wave　velocity　between　phase　B　silicate　and　the　Earth　typical　structure　provide　the　evidence　for　understanding　the　formation　of　the　X-discontinuity　zone　of　the　mantle.