L-systems are powerful rule-based languages for modeling fractal objects and their animation in the fields of virtual plant. But the efficiency of this parallel algorithm needs to be improved because many times rewriting will cost much time and waste lot of computer memory. In this article, an improved algorithm based on the idea of sub-structure is proposed to ameliorate L-systems. Those structures with same features that appear frequently during the process of production iteration are defined as different sub-structures. In the following repetitive process, when the same structure appears, the program will directly paste this sub-structure to the designated position instead of calculating again. Through this method, the storage size of the geometrical information can be decreased and the efficiency of modeling virtual plant can be improved.