Abstract: In this paper, the population dynamics of the parasite Diplostomum spathaceum is studied to understand infections in the hosts Gymnocypris selincuoensis from Siling Lake, including seasonal and group differences in infections and reasons for changes in the population size, identify the parasite’s preferences for male/female hosts, left/right eyes of hosts, or specific parts of hosts’ eyes, and discuss their life history strategies. The samples were collected in different seasons in two years (2020 and 2021), with their total length, weight and sex and the number of metacercariae living on them being recorded. The prevalence and mean abundance of infection of different groups and in different seasons were calculated. To find out whether the parasite exhibits preferences for the hosts, a non-parametric test was conducted for independent groups to identify any significant difference in the size of parasite population on male/female hosts, left/right eyes of hosts and different parts of hosts’ eyes. A total of 165 samples total length: 28.7—49.5 cm; average length: (37.9±4.0) cm; weight: 196.9—827.2 g; average weight: (473.3±127.9) g, including 82 female samples and 83 male samples, were dissected, with 515 parasites being found. The maximum number of parasites observed is 32 worms per fish. In terms of seasonal difference, the prevalence and mean abundance of infection peak in the summer of 2020 and drop in the following autumn and spring. In the summer of 2021, the prevalence of infection is on the decline, while the mean abundance of infection is on the rise. Similar prevalence and mean abundance of infection are observed in the autumn in both 2020 and 2021. To find out group difference, the samples were divided into 5 groups by their total length (TL) at the interval of 5 cm. The minimum values of the prevalence and mean abundance of infection are found in the group of which TL is between 25 cm (inclusive) and 30 cm. Groups of which TL is between 30 cm (inclusive) and 35 cm, between 35 cm (inclusive) and 40 cm, and between 40 cm (inclusive) and 45 cm have similar prevalence and mean abundance of infection. The maximum values of the prevalence and mean abundance of infection are observed in the group of which TL is between 45 cm (inclusive) and 50 cm. The parasite does not exhibit preferences for male/female hosts or left/right eyes of hosts, but they significantly prefers the crystalline lens to the vitreous body. Changes in the size of parasite population are closely related to the seasonal changes, water temperature, bird migration, and snails. The size of parasite population grows with the increase in the total length and eye volume of hosts and frequency of parasite exposure. The reason why the group with TL between 45 cm (inclusive) and 50 cm has higher values of prevalence and mean abundance may lie in the fact that they have a higher risk of parasite exposure because they are larger in size, which enables the hosts to survive from severer infection caused by a larger number of parasites. The preferences exhibited by the parasite in selecting hosts and position of eyes are part of adaptative life history strategies for better transmission and reproduction.