The invention provides a deck support structure and a livestock ship, and involves the technical field of ships. The deck support structure includes a deck cross beam, a deck longitudinal girder and aplurality of support columns; the deck longitudinal girder extends in the length direction of a ship and is supported by the support columns; the deck longitudinal girder has a first web and a secondweb, the first web and the second web are arranged relatively parallelly in the length direction of the ship, and the first web and the second web are perpendicular to a deck respectively and both used for installing the deck cross beam. The technical problems that in the prior art, the support columns are complicatedly connected with the deck longitudinal girder, stand columns of the livestock ship flex badly in a rolling situation and the longitudinal girder inclines to one side are solved.