Abstract: In this paper,stability of plasma under shallow water is systemetically studied,in the way of mutual reaction between water and plasms,First,the phenomena of underwater plasma are thoroughly analysed to find out a corresponding variable used to describe the stable state of underwater plasma, and a stability index of underwater plasma-voltage stability factor is suggested on the base of statistical analysing of the variable. Then,the factors which cause plasma to be uI1stable are pointed out and the methods which irnpldve plasrns stability are researched.