Abstract: 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.