Traditional liquid condensed-phase flame retardants struggle to prepare polyphenylene oxide/polystyrene (PPO/PS) composite foams with high expansion ratios and excellent flame-retardant properties. To address this problem this study investigates the effect of the synergistic flame-retardant mechanism of two solid phosphorus-based flame retardants on the flame-retardant performance of PPO/PS, fabricated using CO₂/ethanol composite foaming technology. The results show that the phosphorus-based compound flame-retardant system exhibits a significant synergistic flame-retardant effect dominated by the gas phase and supplemented by the condensed phase. For unfoamed samples, the maximum limiting oxygen index (LOI) of the sample containing the compound flame retardant reaches 35.8%, which is 4.2% higher than that of the single flame-retardant system, whilst its heat release rate (HRR) remains relatively balanced. The compound flame retardant maintains moderate melt strength, enabling the preparation of composite foams with higher expansion ratios than those obtained with the single flame-retardant system via temperature-rise foaming technology. For a 12% flame retardant addition, the maximum expansion ratio of the compound flame-retardant system increased 16.8-fold. For foamed samples, the LOI values of composite foams with densities greater than 150 kg/m³ are all above 25%； the composite foam with a density of 300 kg/m³ passes the HF-1 grade in the horizontal burning test. Additionally, the compound flame retardant effectively reduces the HRR of the composite foam, thereby improving the dripping phenomenon.