This disclosure discloses a heat pump air-conditioning system and an electric vehicle. The heat pump air-conditioning system includes: an indoor condenser, an indoor evaporator, a compressor, an outdoor heat exchanger, and a first plate heat exchanger, the compressor is in communication with the indoor condenser, the indoor condenser is in communication with the outdoor heat exchanger through a first throttle branch or a first through-flow branch, the outdoor heat exchanger, through a second throttle branch or a second through-flow branch, is in communication with a first branch that is open or closed and is in communication with a second branch that is open or closed, the first branch is in communication with a low-pressure air inlet of the compressor, the second branch is in communication with the indoor evaporator, the indoor evaporator is in communication with a low-pressure air inlet of the compressor, and an enthalpy-increased branch is further disposed in the system, to improve a heating effect in low-temperature heating mode. Hence, effects, such as improving heating energy efficiency and regulatory requirements for defrosting and defogging, can be achieved, and the enthalpy-increased branch can significantly improve low-temperature heating performance of the system.