The invention provides a residual moment measuring device and method of an inertial device based on three-dimensional air flotation, belonging to the technical field of detection. An on-stage system and an under-stage system in the device are connected through a wireless transmission system; the under-stage system comprises an under-stage data collecting and processing system, a server, a cabinetbox and an air supply system; the on-stage system comprises a power supply system, an inertial execution mechanism control system, an inertial execution mechanism to-be-tested product, an on-stage data collecting and processing system, a residual moment test system, a vibration isolating and supporting system, and a vacuum control system, the residual moment test system consists of an air flotation rotary table, a high-precision sensor device and an anti-overturning and protective device, and the high-precision sensor device is separately arranged in a vertical direction of the air flotation rotary table and in a horizontal direction outside the air flotation rotary table. The device can simulate orbit work of a satellite platform, provides a simulated spatial mechanical environment, outputs residual moment in real time, can be used for directly measuring multiple inertial devices, is low in calculation amount, and has high precision.