An automatic drying method for a grain dryer has a preparing step, a parameter-setting step, and a multi-stage drying step. The preparing step includes preparing an automatic drying device (10). The automatic drying device (10) has a body (20), at least two drying sections (30), and a detecting module (40). The detecting module (40) is connected to the body (20) and has at least two moisture meters (41), at least two temperature sensors (42), and a rotary unit (43). The parameter-setting step comprises setting a temperature value and a moisture content of each drying section (30). The multi-stage drying step comprises conveying pre-dried grains (60) into each drying section (30) and drying the pre-dried grains (60) by importing hot air, adjusting an operating speed of the rotary unit (43) and the temperature of the hot air.
