Abstract: By a three-dimensional classical ensemble method, double ionization mechanism of alkaline-earth metal atoms by ultra-low ponderomotive potential fields was investigated, and the higher ponderomotive potential field is compared. The results show that the double ionization process can still be explained by the three-step recolision mechanism at a higher ponderomotive potential fields. However, under the ultra-low ponderomotive potential fields, the double ionization process shows interesting phenomenon. For example, the double ionization yield increases with the increase of atomic ionization energy, which is difficult to be understood by the recollisional mechanism. The trajectory back analysis show that, the collision process takes place in the effective region of the nucleus at ultra-low ponderomotive potential fields, which is called "miniature recollision". Then, the first electron is ionized by the laser field. The second electron usually needs to return to the nucleus several times, and absorb enough energy from the laser field to be ionized. Further sdudies show that "miniature recollision" mechanism plays a dominant role in double ionization at shorter wavelengths, and the recollision mechanism is dominant at longer wavelengths.