In　the　design　of　the　retaining　and　protecting　structures　of　foundation　excavation,　the　theory　of　Rankine＇s　earth　pressure　is　used　to　calculate　the　earth　pressure　on　the　structure　in　the　present　code.　The　method　of　these　design　and　calculation　is　built　according　to　saturated　soil　mechanics.　The　results　of　these　methods　are　not　very　satisfied.　The　equations　of　active　earth　pressure　and　passive　earth　pressure　are　deduced　based　on　the　theory　of　unsaturated　soil　mechanics.　Finally,　taking　the　actual　project　for　example,　the　effect　of　suction　and　construction　time　on　the　earth　pressure　on　the　retaining　and　protecting　structures　of　foundation　excavation　is　studied　according　to　the　equations.