FG series dryer has the operating principle that wet materials are dried at two stages, firstly, raw materials are carried out first-class positive pressure drying by mixed gas formed by tail gas and supplemented hot gas, wherein the tail gas and the supplemented hot gas are of high temperature and low humidity and are carried out by secondary-class drying, after used high-humidity and low-temperature tail gas is discharged out of the dryer, the dried semi-finished products are carried out second-class negative pressure drying by fresh hot air, the dried finished products are metered and packaged, thereby achieving a good circulation drying process, the supplemented hot air can be regulated randomly according to the need, therefore, applicability of the dryer is wide.