A theoretical study was carried out on transient behavior of a cavitating pump system during sudden opening or closure of the discharge valve. Assuming that the transient pump performance is quasi-steady, the system dynamics were determined in a simple fashion for the quick valve opening case. A characteristics method was applied to the transient behavior in the quick valve closure case under the assumption of quasi-steady pump performance. Calculated pressure as well as flow rate at transient agreed well with the measured values. Moreover it is shown that unsteady fluctuations of pressure and flow rate are caused by cavitation behavior during a sharp increase in flow rate, and mainly by a water hammer with water column separation during a sharp decrease in flow rate.