Polyploid strain 149-B, that was generated naturally from a rice twin-seedling population SAR-2, has been determined as triploid (2n = 36). It was then used as the female parent crossing with a normal diploid variety SH R363. From its F2 generation we obtained a genetic-stable population. To prove the uniformity of such a population, SSR markers were used to survey the F2 individual plants. The results showed that F2 individuals carried only one parental molecular marker at each polymorphic locus, and their genotypes were identical with F1 progeny. Based on the above experiments, we consider that this F2 population is definitely an early-generation stable population. Meanwhile, we discussed the possible mechanism of the special phenomenon as well.