The invention discloses a Chinese white poplar lignin monomer synthesis gene 4-CL and an application thereof. The gene is responsible to catalyzing each cinnamic acid and each corresponding coenzyme A ester generated by derivative thereof, the synthesis of each downstream secondary product is participated, and the gene is the essential gene of controlling the lignin synthesis. The findings indicates that the infection of the plant resistance pathogen is reinforced through the increase of the lignin content, the rape stem strength is enhanced, and thus the lodging resistance is obviously enhanced. The concrete result shows that the lignin content of the transgenic rape increases over 10 percents than the acceptor compared with the lignin content; the transgenic plant leaf reduces about 30 percents than the acceptor compared with the disease spots expanding area; and the cane strength of the transgenic plant comparably increases about 20 percents.