We constructed a population dynamic model using demographic parameters with uncertainties by using long-term harvest data and estimated population dynamics of brown bears (Ursus arctos) on the Oshima Peninsula, Hokkaido, Japan, from 1968 to 2021 with population trend indicators obtained by forest sign survey and an upper population limit calculated by extrapolating an estimated density in a high-density area in some years. We assumed mean litter size of 1.8, age of first parturition of 6 years, birth interval of 2.3–3.0 years, and natural mortality of 0.35 for cubs and 0.02 to 0.08 for subadults and adults, with 10% uncertainty for every parameter. The initial female population size was randomized from 1 to 1,809, which was set by extrapolating the 95% value of the female population density (0.327 bears/km²) estimated in 2012 to the entire forested area of brown bear distribution (5,531 km²). This value was used as the upper population limit for 2012. The assumptions for the simulation were that males and females aged ≥6 years were present in 2020, the population size in 1990 when the Spring Bear Removal was abolished did not exceed that in 1968, and the population size in 2012 did not exceed 1,809. Population dynamics differed significantly depending on the demographic parameters; however, the difference in estimation became small and converged after 2012. Continuous harvest and population trends over 40 years maintained the findings within a specific range, even when accounting for parameter uncertainties. Continuous harvest and population trends over 40 years maintained the findings within a certain range, even when accounting for parameter uncertainties. The median population estimates obtained under the conditions of subadult and adult survival of 94%, cub survival of 65%, and birth interval of 2.6 years were 1,107 in 1968; 916 in 1990; 1,715 in 2012; 2,030 in 2021. However, the method outlined in this article has limitations in its application for estimating recent population size and trend. It is crucial to conduct iterated population censuses to ensure accurate and up-to-date data.