Owing to of its freedom from energy inputs, radiative cooling is emerging as a frontier in renewable energy research. Recent advances in nanophotonic structures have enabled scalable passive daytime radiative cooling (PDRC), such as porous polymer films, which can spontaneously reflect solar irradiance and emit heat to the ultra-cold outer space. However, there is still limited fundamental understanding of these nanophotonic structures underlying the cooling effects of these porous polymer-based PDRC films. Hence, we performed optical simulations validated by experiments to quantify how the spatial distribution of nanopores in the polymer film affect its PDRC performance. Our simulation results showed that the influence of spatial distribution of pores on the PDRC performance replies significantly on solar reflectance. A gradient distribution of pores with random pattern can remarkably enhance solar reflectance under direct sunlight up to 153.4 %, while maintaining a slight variation of thermal emittance <4.4 %. With such optimized spatial distribution of pores in polymer, daytime radiative cooling power increased 39.4 W/m2 under direct sunlight in comparison with the randomly distributed pores. These findings indicate that the spatial distribution of nanopores is critical both for demonstrating the cooling effect and for the rational design of nanophotonic structures in high-performance PDRC applications.