A new flame retardant (FR) intumescent system for polybutylene succinate (PBS) was developed using poly(isosorbide carbonate) (PIC), a bio-based polymer, as a ‘green’ char forming agent. The use of this charring agent in PBS/ammonium polyphosphate (APP) permits to obtain better efficiency than PBS/APP alone for mass loss cone calorimetry (MLC) test: pHRR was decreased by 24% vs 19%, THR was decreased by 48% vs 25% and the yield of residue was increased to 44% vs 23% respectively. According to the temperature measured by thermocouples embedded in the sample, the char formed for PBS/PIC/APP showed a better heat barrier effect than that of PBS/APP: under the char, the temperature and the heating rate of PBS/PIC/APP is significantly lower than those of PBS/APP. Obvious differences of the chemical composition and the morphology of the chars highlight that the good results obtained for PBS/PIC/APP were governed by a glass type char formed during the MLC test.