The invention provides a power generation device and method of a liquefied air energy storage system. The power generation device and method are used for adjusting the output power according to power grid needed power. The device comprises a liquefied air energy storage and gasification part, an expansion unit and a power generation unit, wherein the liquefied air energy storage and gasification part is used for storing liquefied air, heating and gasifying the liquefied air, and outputting gasified air; the expansion unit comprises a plurality of air expansion machines, and output shafts of the multiple expansion machines are connected with SSS clutches and are used for converting energy of the gasified air into mechanical power; and the power generation unit is used for converting the mechanical power into electric energy and outputting the electric energy. By means of the power generation method and device, all stages of expansion machines and corresponding auxiliary machines all stably run nearby rated working condition points in the power grid load change process, the liquefied air energy release and power generation efficiency is improved, and meanwhile, safe and stable running of the units and equipment is guaranteed.