Based on the classic plasticity theory， the criterion of seepage stability of unsaturated soil is established by the concept of instability modulus （HIN）. Aiming at the post-failure mode of unsaturated soil slopes （i.e. limited frictional slips and rapid flow slides）， by introducing the elastoplastic suction-dependent constitutive model， the expression of HIN identifying different failure modes of the slope is given. Taking the loose volcanic ash soil with potential liquefaction ability for example， the calibration procedure of theoretical model parameters is introduced. On this basis， the failure mode and its corresponding instability water content of volcanic ash slopes were predicted， thus validating the correctness and rationality of the theoretical model from flume tests. It is worth mentioning that based on the infinite slope model and the theory of instability modulus， the expression of the safety factor of the slope under different instability modes could be derived. Lastly， through parameter analysis， the influence of model parameters on the failure mode of the slope under different boundary conditions is investigated. This study can further clarify the inherent mechanism of different failure modes of unsaturated soil slopes under rainfall conditions， as well as provide theoretical guidance for the evaluation and design of seepage slope stability.