Animal manure is considered to have great potential for phosphorus (P) recovery due to its high P content, while P recovery is limited by the transfer of P from the solid phase to the liquid phase. The conventional dissolution process by adding chemical acid reagents is not economically feasible for animal manure. This study used food waste (FW) as a co-substrate for the anaerobic fermentation of pig manure (PM) to achieve the release of P. The operational parameters were optimized, and the mechanisms of acidification and P release were further studied. The results showed FW promoted lactic acid production and rapid acidification. As FW increased from 0 to 80%, the concentrations of lactic acid rose from 0.12 ± 0.04 to 11.95 ± 1.37 g/L, with pH decreasing from 7.55 to 4.43. The ratio with FW/PM = 1:2 was the optimal condition, which led to the highest soluble phosphate concentration (350.39 ± 8.59 mg/L) in 72 h, with a TP release rate of 74.24 ± 1.81%. Multiple regression analyses established key relationships to predict pH changes in the reactor.