By means of the combinatorial finite elements, the effect of the assembling gap between the contacting faces of the leg of the blade tang and the rim on the strength is analysed. The calculating result indicates that the concentrated stress of the envelope leg is low even if the gap is small. The strength of the leg, therefore, is not a vexing problem.For the short and medium length of the blade the outside displacement of the legs of the rim is so small that it is not possible for the faces of the legs to contact, if there is a wide assembling gap. If there is a small gap retained during the operation of the turbine, the collision between the legs will come into being even if the vibration of the blade is not so serious. It is concluded that the untouchable faces of the legs are not benifical to the damping of the tengential vibration of the blade.