Polylactic acid (PLA), despite its biodegradability, readily degrades into aging microplastics (MPs) in marine environments, while tris(1-chloro-2-propyl) phosphate (TCPP), a representative organophosphorus flame retardant (OPFR), emerges as a concerning marine pollutant. To evaluate their combined effects and ecological risks, mussels were exposed to PLA (both virgin and aging at 0.2 and 1 mg/L), TCPP (0.5 μg/L) and their combination for 14 days, and the gene expressions related to byssus in PLA, aging PLA, TCPP, TCPP + PLA, TCPP + aging PLA treatment were analyzed. Exposed to 1 mg/L aging PLA or 0.5 μg/L TCPP alone suppressed the gene expression related to byssal plaque, respectively. Moreover, the combined effects of aging PLA and TCPP on byssus increased with increasing PLA concentration. Our study emphasizes that, aging would still enhance the ecotoxicity of MPs, even though they are biodegradable. Greater attention should be paid to the risks of PLA in the future.