Abstract: A vehicle heat pump air-conditioning system is provided, which is capable of securing a temperature linearity characteristic during dehumidifying heating, slowing progression of a frost formation on an exterior evaporator during heating, and stably continuing a heating operation while suppressing variations in a blowout temperature. In a cooling refrigeration cycle (14) becoming a base, an interior condenser (8) disposed in a HVAC unit (2) is connected to an exterior condenser (10) in parallel via switching means (15), an exterior evaporator (17) is connected to first decompression means (12) and an interior evaporator (7) in parallel via second decompression means (16), a heating heat pump cycle (18) is configured, the first decompression means (12) and the second decompression means (16) are on-off valve function attached decompression means (12, 16), and the exterior evaporator (17) and the interior evaporator (7) can be simultaneously used during dehumidifying heating and heating.