The invention discloses a thin plastering thermal insulating decorative board and a preparation method thereof. Anchoring parts are pre-embedded in a composite board, and the preparation method mainlyincludes the steps of pre-embedding the anchoring parts, scraping bedding mortar, embedding a gridding cloth, scraping surface mortar, scraping putty, smearing primer paint and finish paint and laminating. More than a dozen steps which traditionally need to be completed by professionals at the construction site are mostly simplified to factory production, thereby reducing the production cost, shortening the construction period and being suitable for large-scale production.