The method of embedded cold-pressing joining could achieve the joining of sheets with different thickness and the joining of dissimilar sheets. Besides, it could meet the lightweight request easily. This article expounded the joining mechanism of this method deeply. To expound the joining mechanism of this method, this article has taken aluminum and stainless steel as examples. In addition, numerical stimulation studies of different hole types’ connecting processes had shown that it was easier to reach plastic yield conditions for aluminum and deform it under the same load conditions. The deformation achieved was significantly greater than that for stainless steel. The width increment of the joining parts increased with the increase of the load pressures; however, the thickness changes of the joining parts behaved in an opposite manner. When the pressing volume of the punch reached 2.4 mm, with the width increment of aluminum sheets of the square hole reaching 2.78 mm, which was the maximum width increment, followed by the hexagonal hole, and the round hole being a minimum. Hole type had a small influence on the width of stainless steel sheets. The experimental results of different hole type joining process indicated that compared with round hole and hexagonal hole, the filling of the square hole was relatively difficult. In case of unreasonable process schemes, defects such as "fake joining," "offset loading" and "hole deformation" would occur. Thus, this article provided a theoretical basis for the scheme establishment and quality control of the embedded cold-pressing joining process.