The present invention provides an operating method of an X-ray CT device, as well as a corresponding device and a system. The present invention acquires the maximum position of the noise index in the length of a scanning area according to the analysis of the acquired plain scanning data of the system, and utilizes a tube current control module to make the noise value calculation within the maximum range and the minimum range of the tube current which the system can provide in the two positions so as to acquires two ranges of the noise index and judge whether the two noise value ranges have intersection. If the judgment is yes, the combination of tube current range and noise index range which meets the requirement is selected within the intersection range and a control system continues the subsequent scanning; if the judgment is no, the system can not meet the requirement of the scanning area and then ends the scanning.