A hydraulic system of an electric vehicle may comprise: an oil container; a first and a second clutch driving circuits connected in parallel; an internal pump driven by a driving system of the electric vehicle; a first check valve connected in series at an outlet side of the internal pump; an external pump driven by an external pump motor; and a second check valve connected in series at an outlet side of the external pump. The first check valve and the internal pump connected in series and the second 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.