The fabrication method of microlens arrays with diameters from 20 to 90 μm based on thermal reflow and dry etch structure transition process is demonstrated. The key process parameters of microlens design and production are obtained by theoretical calculation. And the research of photoresist (AZ4330) surface topography changes during thermal reflow process is studied in detail, which is helpful for microlens array design and process optimization. The photoresist microlens arrays are transferred into quartz by dry etching process. The measurement data indicate that the photoresist microlens arrays have good surface topography and high surface quality, and the optical property of quartz microlens is in good agreement with the theoretical design. This method can be widely used to prepare high quality microlens arrays in optical applications.