In the view of the present problem that the microlens array of high F-number (more than 10) is difficult to be manufactured, this paper proposed a new fabrication method for it. Firstly, A photoresist layer with high viscosity was spin-coated evenly on a microlens array fabricated by melting photoresist and etching technology. Under the viscosity of photoresist and the surface tension of photoresist in curing process, the F-number of microlens array increased observably. Compared with the traditional processing method, this fabrication method is very simple and the surface profile of microlens array is very good. In addition, microlens arrays with different F-number could be obtained with the viscosity of photoresist was adjusted. Based on the method, a microlens array with F-number up to 40 was fabricated. The experimental results show that the profile of the microlens array is good enough to verify the feasibility of the fabrication method.