<p>A bio-based flame-retardant rigid polyurethane foam (FR-RPUF) was developed using a one-shot free-rising method incorporating a bio-based polyol system. The formulation included cyclopentane, a low-GWP, zero ODP blowing agent, and water as a co-blowing agent to ensure optimal foam expansion. The flame-retardant properties of the foam were enhanced through the addition of expanded graphite (EG) and a synergistic combination of ammonium polyphosphate and Piperazine (APP). These additives were integrated into the PU matrix with varying EG and EG/APP ratios to explore their effects on flame retardancy. The flame-retardant performance of the FR-RPUF was rigorously evaluated using multiple testing methods, including burning tests, UL 94 vertical burn testing, and cone calorimetry. Morphological analysis of the foam’s structure was conducted using scanning electron microscopy (SEM), while the chemical composition and functional groups were identified through Fourier Transform Infrared Spectroscopy (FTIR). The results indicated that the foam with an EG/APP ratio of 1:1 exhibited superior flame retardancy, achieving a “V-1” rating in the UL 94 vertical burn test and notable smoke suppression capabilities. Additionally, this formulation demonstrated a compressive modulus of 4.92&#xa0;MPa, underscoring its potential for structural applications where both mechanical strength and flame resistance are critical.</p>