The invention provides a process for preparing glucosamine from chitin through external circulation microwave-assisted acidolysis, and relates to the technical field of preparing glucosamine from chitin. A reaction tank is connected with a microwave heater in a circulating mode, abandoned biomass rich in chitin and inorganic acid are added into the reaction tank for acidolysis, hydrolysate generated through acidolysis is filtered through a microfiltration combined ultrafiltration method so that impurities and macromolecule substances can be filtered out, and then concentration and crystallization are conducted to obtain glucosamine. By means of the process, the acid concentration in the catalysis process can be reduced, energy conservation and emission reduction are facilitated, the problems in the prior art can be effectively solved, continuous and large-scale processing can be conducted, and large-scale industrialized production can be easily achieved.