Abstract: One-pot in situ approach had been employed to prepare Cu-Al layered double hydroxides (CuAl-LDHs). Through controlling the mass ratio (0.82-3.28) of sodium aluminate/ammonium polyphosphate (NaAlO₂/APP), CuAl-LDHs-APP was prepared. XRD, FTIR, SEM and TG were utilized to characterize the prepared CuAl-LDHs and CuAl-LDHs-APP. The fire retardancy and mechanical properties of CuAl-LDHs/polypropylene (PP) and CuAl-LDHs-APP/PP composites were evaluated by limiting oxygen index (LOI), vertical burning (UL-94) test , as well as flexural and tensile tests. SEM observation indicates that the structure of LDHs is sheet-like, with the mass ratio of NaAlO₂ and APP decreasing, the particle size of CuAl-LDHs-APP reduces accordingly, when the mass ratio of NaAlO₂ and APP is 0.82, particle size of CuAl-LDHs-APP reaches about 20 nm, with specific surface area of 183.5 m²/g. TG analysis shows that CuAl-LDHs-APP has preferable thermostability at high temperature. When 20% mass fraction of CuAl-LDHs and CuAl-LDHs-APP is added in PP, the surface of LDHs/PP composites is covered with char. When the mass ratio of NaAlO₂ and APP is no more than 1.64, the addition of CuAl-LDHs-APP can suppress the melting drop phenomenon caused by PP combustion. CuAl-LDHs-APP/PP composite has superior flame retardant and mechanical properties compared with CuAl-LDHs/PP composite. Decreases of mechanical properties of CuAl-LDHs-APP/PP composite such as flexural strength and tensile strength etc. are not obvious compared with PP materials.