The invention provides a lateral flow inclined tube sedimentation separator and a sedimentation tank. The lateral flow inclined tube sedimentation separator comprises at least one sedimentation separation unit, wherein the sedimentation separation unit comprises an inclined plate, a plurality of alar plates arranged at intervals and a plurality of webs. The inclined plate is provided with a guide surface for guiding sediment to move down and a mounting surface arranged opposite from the guide surface. Each alar plate is provided with a first end and a second end which are opposite from each other, and the first end is connected to the mounting surface. Each alar plate is provided with a connecting part located between the first end and the second end. Each web is provided with a first edge and a second edge which are opposite from each other, the first edge is connected to one of two connecting parts, and a second gap for the passage of sediment is reserved between the second edge and the other one of the two connecting parts. Formed isolating areas ensure that both a sediment diffusion area and a muddy water area in a sediment discharge chute are far from a turbulent flow area, moreover, the water of the turbulent flow area is clean water, consequently, muddy water is prevented from adversely influencing sedimentation separation carried out in a sedimentation separation operation area when disturbed by turbulent flow, and thereby the capability of sedimentation and clarification can be improved.