Estimation of sediment erodibility is often required for the evaluation of channel migration, local scour around hydraulic structures, levee stability, etc. Among many erodibility test methods, the constant velocity jet erosion test apparatus is widely used. The premise of this method is the theory of jet flow impinging on a plane surface and the associated wall shear stress. However, in a jet erosion test, the sediment is gradually removed and a scour hole develops as a function of time. As a result, the impinging surface is not planar. In addition, the cylindrical confinement of the jet flow, instead of unbounded in the theory, makes the wall shear estimation departing from the jet theory. This paper uses a 3D computational model to appraise the above effects and quantify the errors by using the theoretical formulas. The computational model is validated by experimental data where PIV measurements were taken in a box without the sediment sample. The use of box is for better illumination of laser sheet of PIV. In another experiment, the sediment sample was placed in the JET apparatus and the scoured bed was scanned at time 0, 8, and 200 minutes. The scanned surfaces of the scour holes are used in the computational model to simulate the flow field at the corresponding moments. It is found that the computational model predicts the flow filed well in the box experiment. Further, the model allows a detailed analysis of the velocity, turbulence intensity, Reynolds stress, etc, in JET apparatus.