Using　the　vertical-torsional　coupling　shear　apparatus,　a　set　of　bi-directional　cyclic　loading　tests　on　saturated　sand　under　isotropic　consolidated　condition　are　conducted.　Based　on　the　existing　definitions　of　dynamic　strength,　the　effects　of　phase　difference　of　bi-directional　dynamic　load　(β)　and　ratio　of　bi-directional　dynamic　load　amplitude　(λ)　on　dynamic　strength　and　pore　pressure　of　sand　are　studied.　The　test　results　show　that　the　dynamic　strength　of　liquefaction　of　saturated　sand　considerably　relates　with　both　β　and　λ.　The　existing　definitions　of　dynamic　strength　have　obvious　limitation.　The　phase　difference　of　bi-directional　dynamic　load　(β)　and　the　ratio　of　bi-directional　dynamic　load　amplitude　(λ)　have　significant　influences　on　the　growth　rate　of　pore　water　pressure,　while　they　have　no　significant　influence　on　the　development　model　for　the　normalized　pore　water　pressure.