Three-phase motor drive system, which generally consists of a three-phase rectifier bridge, a DC bus capacitor and the back stage is an inverter bridge is widely used and its EMI issues becomes more and more critical. Usually, the EMI conduction noise is classified as differential-mode(DM) and common-mode(CM) for EMI analysis and filter design. In this paper, a mixed-mode(MM) conduction noise phenomena that means the DM noise can be induced by CM noise is found due to the abnormal high-frequency turn-on and off of the rectifiers in three-phase rectification bridge in some working conditions, especially when the common-mode noise is larger. The mechanics of the mixed-mode noise is deeply analyzed theoretically and verified experimentally.