This paper describes the modification methods and molding processes in the preparation of biodegradable plastics reinforced with different environmentally friendly biodegradable fillers. Given the distinct chemical structures and properties of different plastics, the same filler can lead to significant variations in material performance. Among the most widely studied plant-based fillers—wood flour, lignin, hemp fiber, bamboo fiber, and starch—were selected due to their abundance, low cost, high development potential, and strong representation in current research. This review evaluates the effects of these fillers on five key biodegradable polymers: polylactic acid (PLA), polyhydroxyalkanoates (PHA), polybutylene succinate (PBS), poly(butylene adipate-co-terephthalate) (PBAT), and polycaprolactone (PCL). The findings aim to serve as a valuable reference for future studies on composite materials.