Cellulosic ethanol has become a hotspot in recent years. However, its crystal structure makes the efficiency of cellulosic degradation by cellulase very low. Traditional ways to disrupt of connection between microfiber consumes a deal of energy and would pollute the environment as well. Plant expansin is known to loosen the plant cell wall, and might provide a synergistic effect on the activities of cellulase. Whereas, the expression level of expansin in plants has been a limit to the functional study and application in cellulose degradation. Thus, it is essential to screen expansin proteins for biomass deconstruction and express them effectively in vitro. Therefore, we cloned expansin gene LeEXP2 from tomato leaves and obtained recombinant Pichia yeast strains integrated with LeEXP2 gene. When incubated in the same culture condition, recombinant strains can secrete the LeEXP2 protein to extracellular medium, while wild-type strain cannot. Preliminary cellulose degradation experiment confirmed that the secreted protein had synergistic the effect of cellulose hydrolysis by cellulase. The experiments of extension strength of filter-paper strips shows that LeEXP2 has a texture-loosening effect on the filter paper, which might make cellulase prone to access cellulose. Above data suggests that LeEXP2 could be expressed effectively in vitro and might become a kind of potential biochemical agent applied in cellulosic biomass conversion for bioenergy production.