Generally, in boundary condition of numerical simulation of internal flow, flow velocity is fixed in the inlet boundary and static pressure is fixed in the outlet boundary or the inverse condition, which is supposed that upstream and downstream pipes and the uniform flows inside the pipes extend infinitely from the boundaries. Therefore, a numerical simulation of influence of pipe length of fluid machinery cannot be realized by changing a boundary position of the computational region. In the present study, a boundary condition which is taking into account a pipe length is used to numerical simulation of unsteady cavitation in three-blade cyclic cascade, where generalized unsteady Bernoulli equation is adapted to one dimensional region from a upstream tank to the inlet boundary. In the study, cavitation surge frequencies reproduced in the present study are compared with empirical and theoretical characteristics of cavitation surge frequencies. Then, it shows good agreement with empirical frequency of cavitation surge although it does not take into account of the system such as pipe length. On the other hand, the theoretical frequency, which takes into account of the system, predicts a value which is less than half of the present numerical result.