A hydraulic system of an electric vehicle, comprising: an oil container; a first and a second clutch driving circuits (L1, L2) connected in parallel; an internal pump driven by a driving system of the electric vehicle, with an inlet thereof being communicated with the oil container and an outlet thereof being communicated with the first and second clutch driving circuits (L1, L2) respectively; a fifth check valve connected in series with the outlet of the internal pump; an external pump driven by an external pump motor, with an inlet thereof being communicated with the oil container and an outlet thereof being communicated with the first and second clutch driving circuits (L1, L2); and a sixth check valve connected in series with the outlet of the external pump, wherein the fifth check valve and the internal pump connected in series and the sixth check valve and the external pump connected in series are connected in parallel between the oil container and the first and second clutch driving circuits (L1, L2); and characterized in that it further comprises a bridge check valve unit having a first check valve, a second check valve, a third check valve and a fourth check valve, wherein the first check valve and the fourth check valve connected in series are connected in parallel with the second check valve and the third check valve connected in series, the bridge check valve unit is connected between the oil container and the fifth check valve, an oil port of the internal pump is connected between the first check valve and the fourth check valve, another oil port of the internal pump is connected between the second check valve and the third check valve. an oil container; a first and a second clutch driving circuits (L1, L2) connected in parallel; an internal pump driven by a driving system of the electric vehicle, with an inlet thereof being communicated with the oil container and an outlet thereof being communicated with the first and second clutch driving circuits (L1, L2) respectively; a fifth check valve connected in series with the outlet of the internal pump; an external pump driven by an external pump motor, with an inlet thereof being communicated with the oil container and an outlet thereof being communicated with the first and second clutch driving circuits (L1, L2); and a sixth check valve connected in series with the outlet of the external pump, wherein the fifth check valve and the internal pump connected in series and the sixth check valve and the external pump connected in series are connected in parallel between the oil container and the first and second clutch driving circuits (L1, L2); and characterized in that it further comprises a bridge check valve unit having a first check valve, a second check valve, a third check valve and a fourth check valve, wherein the first check valve and the fourth check valve connected in series are connected in parallel with the second check valve and the third check valve connected in series, the bridge check valve unit is connected between the oil container and the fifth check valve, an oil port of the internal pump is connected between the first check valve and the fourth check valve, another oil port of the internal pump is connected between the second check valve and the third check valve.