Abstract: In order to solve the problem of poor mechanical properties and thermal stability of hemp straw powder reinforced polylactic acid composite (HPRPC), Pentaerythritol tetrakis3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (PTP) was used to modify HPRPC. The modification mechanism of HPRPC by PTP was investigated by FTIR, mechanical properties analysis, SEM, TG and defect analysis. The results show that PTP enhances the interface compatibility of hemp straw powder (HP) and polylactic acid (PLA), and improves the interfacial properties. When the mass fraction of PTP is 0.6wt%, HPRPC has the best mechanical properties, thermal stability and defects condition. Compared with the untreated composite, the tensile strength and flexural strength of the treated HPRPC are increased by 38.08% and 45.24%, respectively. The mechanical property is significantly improved. And the initial decomposition temperature and maximum weight loss rate temperature are increased by 12.9℃ and 16.8℃, respectively. The thermal stability is improved. And the water absorption rate and the thickness swelling rate of HPRPC are optimized. The water absorption rate of treated HPRPC keeps about 50% of that of the untreated HPRPC. The increase of the thickness swelling rate slows down significantly, and the thickness swelling rate is only 2.11% at 96 h, the defects condition is obviously improved.