Heat pump system and start up control method thereof

A heat pump system comprises: a main heat exchange circuit, comprising a two-stage compressor (100a,100b), a condenser (200), a throttle element (300a,300b) and an evaporator (400), which are connected in sequence to form a circuit; an economizer, disposed between the condenser and the evaporator; a gas supplement branch, connecting a gas outlet of the economizer to a gas supplement port of the compressor, with an economizer regulating valve (600) for controlling the opening and closing of a flow path being arranged on the gas supplement branch; and a control device, wherein the control device controls the opening and closing of the economizer regulating valve based on a refrigerant state feature in the evaporator during a start-up stage of the heat pump system.