The invention provides a method for preparing a light insulating wallboard by light concrete waste. The method comprises the following steps of: smashing and sieving the light concrete waste to obtain light waste particles; uniformly stirring a raw material to obtain concrete, wherein the raw material comprises light concrete waste particles, a gel material, a mineral admixture, a fine aggregate, a coarse aggregate, an additive and water; placing a reinforcing mesh rack in a die for preparing the light wallboard, uniformly distributing stirred concrete to the die, performing steam curing at constant pressure through a vibrating pressure tank, then performing high-temperature and high-pressure maintenance after disassembling the die, and naturally curing to obtain the light insulating wallboard. The invention further provides a light concrete waste recovery device. According to the light wallboard, light concrete waste is used as the raw material, so that not only can the cost be low, but also the problem that the light concrete waste is difficult to process is solved. Therefore, the method is environmentally-friendly. The preparation method is simple in process and suitable for industrial production, and the demand of energy conservation and emission reduction is satisfied by recovering industrial wastes.