1. A current modulation method in a computed tomography imaging system, comprising: selecting a plurality of discrete data points from an expected current modulation curve as approach points, wherein each of the approach points represents an expected current at a view; setting an approach factor for each actual current value, which comprises: selecting from the approach points a plurality of peak points and/or valley points of the expected current modulation curve, sorting the peak points and/or valley points in an ascending order of amplitudes, setting the approach factors in an incremental way for the actual current values at the views corresponding to the sorted peak points and/or valley points, respectively, and setting the approach factors equal to 1 for the actual current values at the views corresponding to the approach points except the peak points and/or valley points; establishing an actual current modulation curve, wherein at each view, the actual current value of the actual current modulation curve approximates the corresponding expected current value of the expected current modulation curve, and a variation rate of the actual current value is within an allowed range of the computed tomography imaging system, thus the actual current modulation curve has an approaching degree to the expected current modulation curve, and the approaching degree is determined by differences between the actual current values and the corresponding expected current values; and modulating a current in an X-ray tube of the computed tomography imaging system according to the actual current modulation curve.