The invention belongs to the field of plant genetic engineering, and discloses an application of a cold-resistant gene PtrERF9 of Poncirus trifoliata in cold-resistant genetic improvement of plants. The gene PtrERF9 is a transcription factor separated and cloned from the extremely cold-resistant Poncirus trifoliata, and has a sequence shown as SEQ ID NO.1. The gene is used for constructing an overexpression and interference vector and is introduced into tobacco, lemon and the Poncirus trifoliata through agrobacterium-mediated genetic transformation, and the biological function verification of the obtained transgenic plant shows that the cloned gene PtrERF9 has the function of improving the cold resistance of the plants. The discovery of the gene provides a new gene resource for plant stress resistance molecular design breeding and provides a new genetic resource for implementation of green agriculture and water-saving agriculture, and the development and utilization of the genetic resource are beneficial to reduction of agricultural production cost and realization of environmental friendliness.