According to the invention, through amplifying a strong promoter in Ensifer adhaerens and carrying out bioinformatics analysis and functional verification, a strong promoter is obtained, and the strong promoter can be widely used for gene expression, genetic manipulation and strain improvement in Sinorhizobium meliloti, Zymomonas mobilis, Caulobacter crescentus, Pseudomonas denitrificans, Agrobacterium tumefaciens, Brucella abortus, Pseudomonas fluorescens, Rhizobium leguminosarum, Ensifer adhaerens and other alpha-proteobacteria and has a nucleotide sequence of SEQ ID NO: 1. The invention also relates to a plasmid vector containing the strong promoter, a method for constructing a genetic engineering strain by using the strong promoter, and an application of the corresponding strain and ahost cell in starting expression of a target gene.