As a micro-optical element, microlens arrays (MLAs) play an important role in imaging, optical communication, lithography, and sensing. This paper reports a new method of fabricating a soft mold of MLAs by surface self-assembly. First, a hexagonal hole array mask is placed on a polydimethylsiloxane (PDMS) film in a radially pre-stretched state, and the masked PDMS film is irradiated using a scanning UV lamp in air, to form a hexagonal array of a silica-like hard film. Because of the surface self-assembly effect, this array bends inward, yielding a soft mold of concave MLAs after the pre-strain is spontaneously released. The focal length of the MLAs could be controlled in the range of 140-330 μm with a 37-μm-diameter hexagonal hole array by varying the exposure time of the ultraviolet ozone. Moreover, the diameter of the hexagonal hole array and the wrinkle wavelength, which are key influencing factors, were systematically studied. The soft mold of MLAs was applied to fabricate replica positive MLAs on planar and curved surfaces, which may have potential applications in image sensing and artificial compound eyes.