An in situ measurement device for sediment critical shear stress in the intertidal zone was developed to accurately obtain the parameters of sediment incipient motion and to reduce perturbation to the undisturbed soil. The inner flow field and the bed shear stress of the device were simulated using the software Fluent. The optimal speed ratio of the inner cylinder and outer cylinder for the most uniform bed shear stress was put forward. The correlations between the rotating speed and the bed shear stress was established and the influence of the device size on the optimal speed ratio was explored. It is found that higher bed shear stress can be obtained when the inner cylinder, outer cylinder and the shear ring rotate in the same direction. A device with an inner cylinder’s radius of 310 mm and an outer cylinder’s radius of 410 mm operating in a water depth of 260 mm was taken as an example. When the inner cylinder and outer cylinder rotate in the same direction without a shear ring, the most uniform bed shear stress distributes in the annular area with a distance of 0. 33~0. 38 m from the axis, and the optimal speed ratio is 3∶1. When the inner cylinder, outer cylinder and the shear ring rotate in the same direction, the most uniform bed shear stress distributes in the annular area with a distance of 0. 33~0. 40 m from the axis and the optimal speed ratio is 3∶1∶3. There is a quadratic function relation between the bed shear stress and the rotating speed of the outer cylinder.