Selective catalytic reduction system and engine system

The utility model relates to the field of engines, and discloses a selective catalytic reduction system and an engine system.The selective catalytic reduction system comprises a heat exchanger, an exhaust inlet, an exhaust outlet, a heat exchange medium inlet and a heat exchange medium outlet; the selective catalytic reducer is provided with an inlet and an outlet, and the inlet of the selective catalytic reducer is connected with the exhaust outlet through a pipeline; the heater is positioned between the inlet of the selective catalytic reducer and the pipeline of the exhaust outlet; the temperature sensor is arranged on one side of an inlet of the selective catalytic reducer; the conversion valve group is arranged at an outlet of the selective catalytic reduction device, and when the conversion valve group is in a first state, the outlet of the selective catalytic reduction device is communicated with the external environment; when the conversion valve group is in a second state, the outlet of the selective