In this work, fly ash (FA), a by-product of coal-fired power plants, was used as the main raw material, with dopamine hydrochloride and tris(hydroxymethyl)aminomethane as modifiers, to prepare a fly ash-based synergist (FA@PDA) via a one-step method. Flame retardant polylactic acid (PLA) was prepared by compounding FA@PDA combined with self-made silicon nitrogen lignin (Si-NLig) and ammonium polyphosphate (APP) to modify PLA. The flame retardant properties, mechanical properties, thermal stability, and combustion behavior of the materials were studied. The results show that the introduction of FA@PDA can effectively enhance the flame retardant performance of flame-retardant PLA, and the LOI value of flame retardant PLA with 2% FA@PDA loading amount can reach 32.0%, accompanied with UL-94 passing V-0 level, which is better than the LOI value 27.0% and V-2 level of flame retardant PLA without synergist. The addition of the synergist improved the mechanical properties of flame retardant PLA, and increased the residual char at high temperature. Cone calorimeter test results show that compared with 8IFR/PLA, the time to ignition (TTI) of 2FA@PDA-8IFR/PLA is extended from 66s to 72s, the total heat release (THR) and peak heat release rate (pHRR) are both reduced, and the fire safety is improved. Flame-retardant mechanism research found that the introduction of FA@PDA promotes flame-retardant PLA to exert good flame-retardant effects in both gas-phase and condensed-phase flame retardancy.