Abstract: Pennisetum purpureum is a perennial resource plant and widely cultivated in tropical and sub-tropical regions. A pair of specific primers was designed to assess the effect of down-regulated expression of the 4-coumarate: CoA ligase (4CL) gene on lignin composition of P. purpureum cv. Huanan, using the 4CL gene sequence of P. purpureum cv. Mott (accession No: GU997597.1). An interference fragment of 374 bp was obtained by PCR amplification with the above primers and it was cloned into the pCR^/GW/TOPO^ vector by TOPO cloning to obtain pENTER-4CL. The corresponding fragment on destination vector pCB2004B was then replaced by the interference fragment of pENTR-4CL by LR reaction to form the RNAi (RNA interference) expression vector pCB2004B-4CL. The Agrobacterium-mediated transformation of the expression vector was carried out using the freeze-thaw method, and the expected fragments appeared in the PCR products using bacterial liquid as a template. This study was a basis for research on the 4CL function of P. purpureum by RNAi technology.