The effect of Jet-Pump in the transient procedure of natural circulation and the resistance behaviour of Jet-Pump in the different flow condition have been analyzed.

Jet pump may be all kinds of material, such as cast iron, steel, stainless steel, Hastelloy, titanium, ceramic, glass, plastics and graphite, and made of various sizes.

The unsteady flow in a jet pump is calculated and the reasonable calculation result was gotten based on above research.