The frequency stability of the frequency stabilization system is dependent on the loaded figure merit of the resonant cavity. To increase the figure merit infinitely is not correct because the power spectrum has a （siny/y）2 form for the pulse magnetron which is the power source of a LINAC. From the Parseval theory

area of the output signal of the square-law detector terminatied by the cavity will be directly proportional to output energy of the cavity so that the variation of the signal area is equivalent to that of output energy as oscillation frequency of the magnetron is changing. On the basis of this principle

a recommended expression and the upper limit of the figure merit are given.