Using cotton stalk lignin (LG) as raw material, three kinds of urea-based modified cotton stalk lignin (UM-LG), esterified lignin (EL) and m-phenylenediamine-based modified cotton stalk lignin (ML) were prepared. A modified cotton stalk lignin was added to polypropylene (PP) as a polymer modifier to study the effects of different modified lignins on the flame retardant and antioxidant properties of PP. The results showed that the three kinds of modified lignin improved the flame retardant and antioxidant properties of PP. After adding 15wt% UM-LG to PP, the oxidation induction time was increased by 52.9% compared with the pure PP sample; the shear torque was increased by 50.58% at 200 °C, screw speed 30 r/min, and reflux for 10 min. However, after adding 15wt% ML to PP, the oxygen index reached 24.9%. It can be found in the SEM image that the addition of three modified lignins made PP form a carbon layer during the combustion process, which further demonstrated that the modified lignin enhanced the PP Flame retardant properties. The mechanical properties test showed that: with the addition of modified lignin, the mechanical properties of PP were affected to a certain extent.