The group of Cerasus subhirtella has a high ornamental value with rich variations. Disputes exist regarding its classification. In this experiment, taking tomato as an internal standard, we used flow cytometry (FCM) to determine the genome size of tomato and other samples of C. Subhirtella. By comparing the multiple relationship peak of tomato and other samples of C. subhirtella, the genome sizes of different C. subhirtella samples were calculated. After several experiments, the results showed that the genome size of C. subhirtella was about (512.260 ± 16.200) Mb, C-value about (0.54 ± 0.289) pg. The genome size of C. subhirtella var. ascendens was (274.360 ± 13.920) Mb, C-value about (0.28 ± 0.014) pg (1 pg = 978 Mb). Such a difference in genome size supports that C. subhirtella var. ascendens is another variety.