The present disclosure includes: in a collimation condition, performing air scan based on a predetermined reference scanning condition to obtain air calibration data corresponded to the predetermined reference scanning condition and obtaining air calibration data of non-reference scanning conditions in the collimation condition based on the air calibration data corresponded to the predetermined reference scanning condition and stored incremental differences, wherein the stored incremental differences are differences between the air calibration data of the predetermined reference scanning condition and the non-reference scanning conditions.
