Aiming at the disadvantage of traditional test method for residual flux factors, this paper proposes a bidirectional DC method to measure residual flux factors. It omits the procedure of demagnetization before test, adopts positive excitation, de-excitation, negative excitation, and excitation on electromagnetic CT, collects relevant excitation voltage and current signals, and calculates the change of magnetic flux during excitation and thus obtains residual flux factors through test. Based on the DC method, a residual flux measuring instrument is designed. The comparison test and uncertainty analysis show that bidirectional DC test method based residual flux measuring instrument can accurately measure the residual flux factors of electromagnetic CT.