Disclosed is a vehicular HVAC system, including: a battery circulation line including a battery radiator, a high-voltage battery, and a first valve, arranged to allow a first coolant to circulate therethrough; a branch line selectively connected to the circulation line through the valve and having a cooling heat exchanger configured to exchange heat with an indoor air conditioning refrigerant line; a battery heating line including a water heater, an indoor air conditioning heater core, and a second valve, arranged to allow a second coolant to circulate therethrough, the battery heating line being selectively connected to the circulation line through the second valve; and a controller configured to control the first valve in stages, to cool the high-voltage battery, to control the second valve and the water heater to selectively heat the battery, and to control the water heater or a cooling device on the refrigerant line to air condition the vehicle.