A method for determining a longitudinal position of a combined phantom, a computer readable storage medium, and a CT device. The method includes: scanning the combined phantom to obtain a topogram thereof. The combined phantom is fixed to a table top, protrudes beyond the table top, is axially parallel to the longitudinal direction, and includes a plurality of phantoms, one of which is a slice phantom. The method further includes identifying the plurality of phantoms on the topogram, and using an axial center of the slice phantom as the longitudinal position of the combined phantom. The longitudinal position is determined by using the topogram of the combined phantom, and a phantom position is aligned automatically.