The invention relates to a mounting forming method of a heat insulating layer, belonging to a manufacturing method of a heating insulating layer for a solid rocket engine. The mounting forming method mainly comprises the following steps: a preparation step, a gluing step, a locating step, a mounting step, a thermal pre-compacting step, a flaw detecting step and a curing step, thus forming the heat insulating layer in an engine housing. Since the forming of the heat insulating layer is carried out in vacuum, the debonding defect caused by gas is avoided and problems causing poor product quality for a long time as interface debonding and interlayer gas entrapment can be solved completely; all the bonding operations are implemented in a vacuum mounting mode; manual operation by experience in the prior art is improved into a quantitative control; individual difference in product quality can be eliminated basically; the molded surface of the heat insulating layer is ensured by a mold, thereby greatly improving the accuracy of the molded surface of the heat insulating layer, improving working efficiency and ensuring production quality.