Flexible mattress constructed with unidirectional grille is a new type of flexible mattress, which is composed of three parts from the bottom to the top:anti-filtering geotextile fabrics, unidirectional grille, and concrete ballast block. Taking the revetment project in the middle and lower reaches of the Yangtze River as the application scenario, the finite-element method is used to simulate the stress characteristics of the unidirectional grille constructed flexible mattress under the condition of downstream sinking. The maximum stress of the unidirectional grille constructed flexible mattress increases with the relative displacement of the ship, the water depth, the water velocity, and weight of ballast block, while the maximum stress decreases with the increase of the angle between the water flow direction and the flexible mattress sinking direction. Under various downstream sinking conditions, the stresses obviously concentrate at the lower end of the flap of the paving ship on the unidirectional grille constructed flexible mattress, and the maximum stress is located at the lower end of the flap and about one meter away from the edges on both sides in the width direction of the flexible mattress, while the middle part of the flexible mattress is less stressed. The results show that the maximum stress of the unidirectional grille constructed flexible mattress is much lower than its tensile strength, and it presents good adaptability in the riverbed protection project of the middle and lower reaches of Yangtze River. Compared with the traditional flexible mattress made of the reinforced belt, the cost of the unidirectional grille constructed flexible mattress can reduce by more than 20%.