The invention discloses a fireproof and insulating light composite glass magnesium board and its preparation method. The board is prepared by the following raw materials by weight: magnesium chloride hexahydrate, a modifier, light calcined powder, calcined forsterite, forsterite or serpentine raw ore. The method comprises the steps of: 1. using magnesium chloride hexahydrate to prepare a solution, and adding a modifier so as to form a liquid mixture; 2. grinding forsterite raw ore powder and calcined ore powder, and mixing the powder with light calcined powder, then stirring them well; 3. subjecting the mixed powder into a double-shaft paddle mixer for mixing, stirring, and metering; 4. delivering the well stirred material into a template, which is automatically placed on a template processing roller platform by means of a plate-sucking machine, then conducting pouring, wrapping a reinforced glass fiber cloth, then conducting rolling, thickness setting and moulding; 4. sending the moulded plate to maintenance, and putting the product plate in a trimming machine for shaping, performing inspection, thus obtaining a qualified product. The method of the invention has simple process and convenient operation, and is conducive to implementation. The glass magnesium board provided in the invention has the advantages of high efficiency insulation, plate structure, no thermal bridge andcold bridge, excellent heat insulating performance, flame resistance and weatherability, as well as excellent flame retardancy and refractability.