In this study, based on bioinformatics analysis, a WRKY transcription factor gene was cloned from Stylosanthes guianensis (Aubl.) by RTPCR and named as StWRKY45. The largest open reading frame (ORF) of this gene is 924 bp, encoding 307 amino acids ,with a molecular mass of 35.62 kD and an isoelectric point of 9.81. Phylogenetic tree analysis indicated that StWRKY45 belongs to the WRKY gene of WRKY transcription factor class Ⅱ, and has the closest relationship with Arabidopsis AtWRKY45 and AtWRKY57. The results of Realtime PCR showed that under the nonphosphorus stress (control), StWRKY45 gene was expressed in roots, stems and leaves of S. guianensis seedlings, but the expression level was relatively low; under low phosphorus stress, the expression of StWRKY45 gene in roots, stems and leaves increased gradually with the prolongation of stress time, and the expression level was the highest in leaves. The relative expression of leaves and stems reached the highest at 96 h after low phosphorus stress. They were 20.47 times and 9.38 times of the control, respectively, but the relative expression in roots reached the highest at 72 h after low phosphorus stress, which was 9.29 times of the control. Studies have shown that StWRKY45 gene is induced by lowphosphorus stress, suggesting that StWRKY45 gene may be involved in the response of S. guianensis to low phosphorus stress.