Abstract: This paper discusses the simulation of weak plane by means of four-node weak plane element with uniform thickness, laying emphasis on the influence of turning rock mass to the weak plane element. No matter how small the thickness of the weak plane is, the great influence to shear stress is produced due to rock mass turning if the element of the weak plane is turned round the end of the element, even the positive and negative symbols of the shear stress of element are changed due to rock mass slipping. A set of mathematical formulas are well established in the paper, including the geometric matrix Be, stress matrix Se and stiffness matrix Ke of the elements referring to turning influence as well as the equivalent nodel load equation due to initial stress. It is proved that Goodman's joint element is a special case of four-node weak plane element with uniform thickness whenδ/1= 0.