The invention relates to a protective plate which can effectively protect a waterproof material from being aged and damaged by other external force and ultraviolet light and a preparation method thereof. The protective plate of the waterproof material contains the following formula components by weight percent: 6-19% of alkaline stone material, 7-21% of mineral powder, 2.5-6.5% of lignin fiber, 2.5-8% of glass fiber mat and the balance of construction asphalt. The protective plate prepared according to the special preparation method has the advantages of high compactness, good flexibility, compaction resistance and strong interlayer adhesive force, and can effectively protect a waterproof layer from being damaged by the external force and prolong the service life of the waterproof material.