Abstract: The thrust of an ion thruster is proportional with extracted beam current and determined by its magnitude.For ion thruster with stable working parameters，beam current decreases as time elapse.In order to find out the definitiveness factors causing the problem，The research on ion thruster by analyzing fault tree of all causing factors is illustrates.Main factors were found out by investigating and verifying the bottom events of fault tree.It summarizes the research that definitiveness factors causing beam current decreasing are grids characteristics，magnetic field characteristics and hollow cathode characteristics，of which hollow cathode characteristics is the main causing factor.