All-day passive radiative cooling has recently attracted broader attention for its potential as a viable energy technology. Although tremendous progress has been achieved, the design and fabrication of low-cost high-efficiency radiators for all-day passive radiative cooling remains a challenge. Herein, we report a new type of flexible composite radiator film with built-in artificial opal-like structures for all-day passive radiative cooling. Using artificial opal structure concepts, the proposed polydimethylsiloxane (PDMS) radiator film with embedded polystyrene (PS) microsphere photonic crystals exhibits a sufficiently high solar reflectance of ∼92.7% when in a direct sunlight region, and a thermal emittance of ∼93.6% within the atmospheric window. Without the need for traditional reflectors like silver or aluminum foils, this composite film realizes subambient temperature reduction of ∼4.8 °C in direct sunlight and ∼8.5 °C during the night. This work provides a new fabrication approach for the low-cost production of structural polymer films for high performance and potential real word applications.