An engine system for controlling an exhaust gas flow includes an intake line arranged to draw in outdoor air, an engine combusting the outdoor air supplied through the intake line and fuel in a combustion chamber of the engine to generate torque, an exhaust line for exhausting exhaust gas from the combustion in the combustion chamber of the engine, a turbocharger having a turbine operated by the exhaust gas passing through the exhaust line and a compressor for compressing the outdoor air in the intake line, a catalyst unit arranged on a downstream side of the turbocharger for reducing harmful components of the exhaust gas, a bypass line branched from the exhaust line on the downstream side of the turbocharger and joined to the exhaust line on a downstream side of the catalyst unit, and a bypass valve arranged on the bypass line for selectively opening/closing the bypass line.