A multi-loop natural circulation experimental device under six-degree-of-freedom motion conditions and a method therefor are provided. The device includes: a six-degree-of-freedom motion simulation platform; a multi-loop main circulation loop including a serpentine pre-heater, an experimental section, two sleeve-type condensers, and a pressurized circulating pump, a voltage stabilizer and related equipment; and a cooling water system including a sleeve condenser, a plate heat exchanger, a cooling tower, a cooling fan, a cooling water tank and related equipment; and an electric heating system including a DC power supply, a low voltage power controller and a transformer. The present invention also provides an experimental method of the device.