Abstract: To investigate the differences in pore structure with different particle sizes when coal body immersed in different amounts of water, coal samples with three particle sizes and four degrees of immersion were prepared using fat-coal (FM) and coking coal (JM). The low-temperature nitrogen adsorption experiment was conducted to determine the pore structure of coal, and fractal theory was employed to accurately quantify the pore structure. The study analyzed the differential effects of water immersion degree and particle size on coal pore structure. The research results show that under the same degree of immersion, as the particle size decreases, the specific surface area, single-layer saturated adsorption capacity, total pore volume, and isothermal adsorption capacity of the coal sample all show an increasing trend, and the change in specific surface area is the most obvious. The BET specific surface area of coal samples with particle size of 0.15-0.18 mm is about three times that of coal samples with particle size of 0.25-0.85 mm. The single-layer saturated adsorption capacity is positively correlated with the change in specific surface area, and the growth of JM is particularly significant at the minimum particle size. The analysis of total pore volume shows that the pores are mainly mesopores, and after water immersion, some coal samples exhibit the phenomenon of large pore collapse and an increase in mesopores, resulting in a decrease followed by an increase in pore volume. The isothermal adsorption curve shows that the adsorption process conforms to the typical characteristics of mesoporous materials, and the adsorption capacity of coal samples with the smallest particle size (0.15-0.18 mm) is always the highest. The effect of immersion has a dual nature: at low immersion rates, water occupies large pores, leading to structural collapse and a decrease in specific surface area and single-layer saturated adsorption capacity; when the immersion water is high, the dissolution effect promotes the secondary development of pores, and the relevant parameters increase accordingly. The fractal characteristics further indicate that the fractal dimension changes of coal samples after water immersion do not follow a single law, but are jointly regulated by the degree of immersion and particle size. The degree of immersion and sample size are key factors leading to differentiated changes in coal pore structure.