The colloidal system consisting of two large spheres confined in a cylindrical pipeline and immersed in the sea of small spheres is considered in this paper. In this system，the depletion interactions between the large spheres and the wall of cylindrical pipeline are studied by Monte Carlo simulation and acceptance ratio method. The numerical results show that the depletion forces are coupled，i.e.，the depletion force between the two large spheres is strengthened by the depletion force between one of the large spheres and the wall of the cylindrical，meanwhile，the depletion force between one large sphere and the wall of the cylindrical is also strengthened by the depletion force of the two large spheres. Furthermore，through the numerical results，we found that the coupling effect is increased with the decrease of the diameter of the cylindrical pipeline.