Based on the superconducting quantum circuit system,we propose a quant um non-demolition measurement for preparing Fock states.In the case of coupling with the cavity,the two-level superconducting qubit formed by Josephson structure will produce a Stark shift, which is proportional to the number of photons in the cavity.Based on the property that the energy level shift of superconducting quantum bit is affected by the photon in the cavity,th e number of photons in the cavity can be determined by the measurement of the photon transmittance i n the waveguide when the waveguide is coupled with the superconducting qubit.This measurement m ethod has no influence on the photons in the cavity,so it is a quantum non-demo lition measurement,and then the quantum states of the photons in the cavity can be prepared.In addition,we also di scuss the influence of relative parameters on the preparation effect.Based on this,we propose a me thod of feedback repeated measurement to improve the preparation efficiency and realize the rapid preparation of the target quantum state.