The invention relates to a deepwater substrate sludge sampling device comprising a grab bucket, a lifting hook and a lifting hook support mechanism, wherein the grab bucket is composed of a left half grab bucket and a right half grab bucket. The lifting hook support mechanism comprises a lifting hook support, a left lifting frame, a right lifting frame and a steel cable, when deepwater substrate sludge is extracted, upper horizontal support rods on the left lifting frame and the right lifting frame are respectively hooked by a left hook and a right hook of the lifting hook, when the left halfgrab bucket and the right half grab bucket touch the deepwater substrate sludge, the lifting hook is separated from the left lifting frame and the right lifting frame, the lifting hook support is lifted up, is fastened when being pulled by the steel cable so as to sample sludge and lifts the sampled sludge to a water surface. The deepwater substrate sludge sampling device provided by the invention can guarantee that the grab bucket (the main component of the substrate sludge sampler) can normally operate in a launching process and a sludge sampling process can not be influenced as the grab bucket is closed in the launching process, and the deepwater substrate sludge sampling device provided by the invention has a simple structure and less components, is easy to popularize, is convenient to use, has small volume and is easy to carry.