To investigate the possibility of phosphorus (P) recovery from marine sediment and explore the role of the carbon: nitrogen ratio in affecting the internal P release under anaerobic conditions, we experimented with the external addition of carbon (acetic acid and glucose) and ammonia nitrogen (NH4-N) to expose P release mechanisms. The 24-day anaerobic incubations were conducted with four different carbon: nitrogen dosing groups including no NH4-N addition and COD/N ratios of 100, 50, and 10. The P release showed that extra NH4-N loading significantly suppressed the decomposition of P (p < 0.05) from the marine sediment, the maximum P release was 4.07 mg/L and 7.14 mg/L in acetic acid- and glucose-fed systems, respectively, without extra NH4-N addition. Additionally, the results exhibited that the imbalance of carbon: nitrogen not only failed to induce the production of organic P mineralization enzyme (alkaline phosphatase) in the sediment but also suppressed its activity under anaerobic conditions. The highest enzyme activity was observed in the group without additional NH4-N dosage, with rates of 1046.4 mg/(kg∙h) in the acetic acid- and 967.8 mg/(kg∙h) in the glucose-fed system, respectively. Microbial data analysis indicated that a decrease in the abundance of P release-regulating bacteria, including polyphosphate-accumulating organisms (Rhodobacteraceae) and sulfate-reducing bacteria (Desulfosarcinaceae), was observed in the high NH4-N addition groups. The observed reduction in enzyme activity and suppression of microbial activity mentioned above could potentially account for the inhibited P decomposition in the presence of high NH4-N addition under anaerobic conditions. The produced P-enriched solution from the bioreactors may offer a promising source for future recovery endeavors.