To investigate the effect of strain rate on spall strength of Ti-6Al-4V dual-phase alloy, the spall processes of Ti-6Al-4V dual-phase alloy under different impact velocities were simulated by molecular dynamics method. It was found that the spall strength increased first and then decreased with increasing strain rate. The critical strain rate ( ${\dot{\epsilon }}_{\text{c}}$) for the transformation of spall mechanism from classic spall to micro-spall was proposed and verified for the first time. The classic spall occurred when $\dot{\epsilon }\le {\dot{\epsilon }}_{\text{c}}$, while the micro-spall occurred when $\dot{\epsilon }>{\dot{\epsilon }}_{\text{c}}$. It was revealed that the internal mechanism for the transformation of spall mechanism from classic spall to micro-spall was closely related to dislocation density and melt ratio. The voids nucleated under the tensile stress produced by the meeting of two opposite rarefaction waves when the classic spall occurred. The increase of dislocation density led to the increase of spall strength. The voids nucleated under the stress-thermal synergistic effect of tensile stress and high temperature melt when the micro-spall occurred. The increase of melt ratio led to the decrease of spall strength.