Chromosome doubling is considered an important technique in poplar breeding, with many triploid clones being artificially induced and selected for promotion in the north and northeast of China because of their outstanding traits in vegetative growth and environmental adaption. In this study, the triploid yield of Populus simonii Carr × P. nigra var. italica (Moench.) Kochne was 23.41%, which exceeded the yield attained in our previous studies due to the use of an optimized method of chromosome doubling in the embryo sac at a high temperature. The development of the embryo sac after the pollination of this hybrid was investigated to determine the induction period. Ploidy of seedlings was identified by flow cytometry after initial filtering using the chloroplast counting method. Eleven triploids and one tetraploid were ultimately obtained, and the optimal operating conditions were exposure of female catkins to 41 °C for 2 h at 66 h after pollination (HAP). This study identified an efficient method of chromosome doubling in P. simonii × P. nigra var. italica and provided several polyploids for Populus polyploid breeding programs and subsequent studies.