The sucking outflow phenomena of a passive piezoelectric pump under the square wave driving is studied and, a hydrodynamics model and a vibration model of passive piezoelectric pump are built to analyze theoretically the physical process of the sucking outflow process.Then,an open height formula of the existing valve in sucking processing is derived. The open height of the piezoelectric pump is measured during the sucking outflow process under different frequencies of 0.4,0.8,6,45 Hz, and the other factors effecting on the sucking outflow in the formula are verified. In the theoretical analysis,it points out that the flow inertia is the root cause for the outflow sucking phenomena, and the inertial flow also exists during the draining process. The experiment indicates that when the piezoelectric frequency increases, the open height of the output valve during sucking process decreases. When the exiting valve rigidity and the loadings in input and output increase, the outflow decreases.The results above are coinciden with the formula well. It concludes that the output flow of the piezoelectric pump should include two parts: cubage flow and inertial flow, which should be considered in the same time when the flow of the piezoelectric pump is controlled precisely.