The invention relates to a high-accuracy measurement technical field of current amplitude and phase, in particular to an online mark and check method for the transformation ratio and angle difference of current transformer CT, and a relative system, wherein lg(t) is the sensitive output of the current transformer with linearly mixed lx(t) and lx(t), therefore, Ig(t)=1Ix(t)+a2IS(t), while lx(t) is the object current, ls(t) is the current signal generated on the measurer, a1 and a2 are the sensitive factors of lx(t) and ls(t) on the output signal lg(t) of the current transformer, then the invention uses independent element analysis method to separate signal ls(t) from signal lg(t), to obtainthe evaluated signal ls'(t) of ls(t), and via the mixed factors a1 and a2 to obtain the transformation ratio and angle difference or the like of current transformer in real condition. Via comparing ls'(t) and ls(t), the invention can easily and reliably realize online check of current transformer and check the result of the independent element analysis.