P chromosomes may carry a genetic system that inhibits the Ph gene in wheat. Abnormal chromosome synapsis in wheat-Agropyron cristatum addition line II-21-2 (additional 1·4 recombinant P chromosome) was observed in this study. The results of cytogenetics and Ph1 gene amplification showed that the Ph1 gene was normal and the average number of quadrivalents or hexavalents was determined to be 0.41 and 0.13, respectively, in pollen-mother cells of wheat-Agropyron cristatum addition line II-21-2. The analysis of dual-color GISH/FISH showed that the P chromosomes were not directly involved in the composition of multivalents but could inhibit the effect of the Ph gene, leading to synapsis of wheat homoeologous chromosomes and translocation between wheat homoeologous chromosomes such as 3B-3D chromosomes. The characteristic of P chromosomes’ promoting synapsis of wheat homoeologous chromosomes may have potential application in the genetic improvement of wheat. P chromosomes may inhibits the Ph gene in wheat. Abnormal chromosome synapsis in wheat-Agropyron cristatum addition line II-21-2 (additional 1 · 4 recombinant P chromosome) was observed in this study. The results of cytogenetics and Ph1 gene amplification showed that the Ph1 gene was normal and the average number of quadrivalents or hexavalents was determined to be 0.41 and 0.13, respectively, in pollen-mother cells of wheat-Agropyron cristatum addition line II-21-2. The analysis of dual -color GISH / FISH showed that the P chromosomes were not directly involved in the composition of multivalents but could inhibit the effect of the Ph gene, leading to synapsis of wheat homoeologous chromosomes and translocation between wheat homoeologous chromosomes such as 3B-3D chromosomes. The characteristic of P chromosomes’ promoting synapsis of wheat homoeologous chromosomes may have potential application in the genetic improvement of wheat.