The surface shape of structure plane is closely related with its mechanics property. Precise measurement to the surface shape of structure plane is the basis of deformation mechanism and strength characteristics. On the basis of studying advantages and disadvantages of the measuring instrument, the author has developed a new kind of portable measuring instruments innovatively - Contact Punching Device which has improved open-air working efficiency greatly. Application results show that JRC obtained by the Contact Punching Device is very close to the actual JRC of surface shape of the structure plane. Then about size effect of rigid structure plane, got the following principle that the statistical variance of the undulating roughness of the structure surface decays gradually with length of the rock sample, and got the limiting length of the undulating roughness when the influence of shearing intensity of the structure is dispelled. Finally, which proved through the embodiment of different rock and different weathering rock, the result indicates this method can be applied to engineering.