In a library’s reading area, sufficient light exposure is needed to see objects around the users’ field of vision for their activities. Regarding the case study, as an equatorial region, Indonesia also has the advantage of getting a continuous high radiation climate throughout the year. Therefore, buildings around the area need to optimize this benefit to achieve optimal energy efficiency. Meanwhile, according to the national standards of SNI 6197-2011, the simulation of the existing reading area in PGRI University’s Library has not achieved the optimized daylighting performance. This research aims to improve the lack of daylighting performance, using DIALux EVO simulations as the primary method to analyze the library’s existing condition and to evaluate each design proposal for retrofitting. The study then found that among all other options, the recommendation to add five more window openings on the north walls, setting up 0.8-meter width light shelves, and reducing the window overhang length to 0.25 meter is the most effective ways to optimize the daylighting performance in the library.