Eu(DBM)3Phen-doped polymer optical fibers (POFs) were prepared by thermal polymerization. The cross relaxation model of clustering Eu3+ ions was described, and the clusters in this kind of fibers were studied based on the above model. The percentages of clustering ions in different fibers were obtained by the rate equation combining with fiber transmission experiment. While concentrations in these fibers are 0.1 wt.-%, 0.2 wt.-%, 0.3 wt.-%, 0.4 wt.-%, 0.5 wt.-% and 1 wt.-%, the percentages of clustering ions are 0.045, 0.07, 0.07, 0.07, 0.07 and 0.08, respectively. The result shows that this kind of fibers has low percentage of clustering ions. With the increasing of Eu3+ concentration, the percentage of clustering ions almost not increases, the clustering of Eu3+ ions is not obvious.