One set of one-dimensional unsteady homogenous mathematic models for flow and heat process of natural circulation precooling loop of cryogenic liquid rocket engine was established

whilst both inverted annular model and dispersed model were introduced for the prediction of filming boiling region.The results indicate the unstable characteristics of circulation flow rate in precooling process is caused by the unstable characteristics of natural circulation driving force

that is

the heat flux transferred to the cryogenic liquid from the natural circulation precooling loop.Moreover

about 80% of wall temperature drop takes place in filming boiling region

and the inverted temperature distribution law of the return pipe is well interpreted by introducing the two filming boiling models mentioned above.