The utility model provides a structure design of a horizontal dynamic autoclaved reactor for producing alpha-hemihydrate gypsum. The reactor consists of a reactor body (1), a steam jacket (2), a jacket steam inlet pipe (3), a stirring center shaft (4), a feeding hole (5), a steam outlet pipe (6) in the reactor, stirring paddles (7), a sealing part (8), supporting legs (9), a condensed water outlet pipe (10), a discharging hole (11), a steam outlet (12) of the center shaft and a steam inlet (13) of the center shaft, wherein the reactor body (1) consists of a steel cylinder and sealing covers (1A) at two ends of the steel cylinder; and reactor can directly utilize the industrial by-product gypsum which does not need to be pressed in block shape for further processing or to be mixed with a large amount of water in the reactor for preparing a suspension. The reactor has the obvious advantages of inside and outside double heat transfer, dynamic stirring, high heat transfer strength, high productivity, short reaction time, high product strength and stable quality.