ABSTRACTEsterified starch was used as an interfacial modifier to treat the surface of cassava dregs. The treated fiber was used to prepare flame‐retardant poly(butylene succinate) (PBS)/cassava dregs fiber composites with the incorporation of intumescent flame retardants (IFR). The mechanical performance and flame‐retardant properties of composites were investigated. Experimental data showed that an appropriate cassava fiber loading favored the mechanical performance of composites. When the total filler content was 30 wt % [m(cassava dregs):m(IFR) = 1:5], in comparison with those of composite prepared by 30 wt % IFR, the tensile and impact strengths of composite increased by 40 ± 7 and 62 ± 8%, respectively. Besides, the limited oxygen index value of 37.3% and UL‐94 V0 rate of composite could be achieved. Possible flame retardant mechanism was proposed. The combusted residue of incorporated cassava dregs could play a support effect in the three‐dimensional charred layer formed by the combustion products of IFR and PBS. The three‐dimensional intumescent charred layer, and the formation of incombustible gas, such as NH3, play an important role in insulation, oxygen barrier, thereby effectively improving the flame retardancy and thermal stability of composites. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46210.