The invention discloses a desulfurized gypsum base machine-sprayed thermal insulation mortar and a preparation method thereof, and relates to the field of composite thermal insulation materials. The mortar comprises the following components in parts by weight: 65 to 85 parts of calcined desulfurized gypsum powder, 5 to 10 parts of cement, 10 to 20 parts of slag micropowder, 0 to 2.5 parts of thixotropic lubricant, 0.8 to 2.2 parts of EPS particle, 0.4 to 1 part of redispersible emulsion powder, 0.2 to 0.8 part of water retaining agent, 0.2 to 0.8 part of alpha-starch, 0.05 to 0.35 part of gypsum retarder, 0.1 to 0.5 part of polyvinyl alcohol, and 50 to 80 parts of water. The preparation method is simple, the cost is low, and the thermal insulation mortar has a good thermal insulation performance, is environment-friendly and energy saving, and is suitable for machine spraying operation.