Abstract: Expansive soil is a type of highly plastic clay, where the volumetric expansion upon water absorption (hygroscopic expansion), while the shrinkage upon water loss. This property can be detrimental to the soil structure, resulting in a series of civil engineering problems, such as soil surface cracking, subgrade settlement, and road surface uplift. Alternatively, coal gangue is a kind of rock waste discharged from coal mining, washing, and processing during coal production. Serious influences have occurred in the social, environmental, and economic respects. An eco-friendly way to save energy is to effectively reuse the coal gangue as a substitute resource. This study aims to improve the properties of expansive soil using the coal gangue, in order to alleviate the damage of expansive soil in engineering, thereby to explore the porosity characters of improved expansive soil with coal gangue. A Menger sponge model was selected to characterize the fractal dimension of pores, in order to guide the development of curing agent, and thereby control the pore structure of expansive soil via the fractal characteristics. A mercury injection test was performed on the samples of expansive soil with the content of coal gangue of 0, 3%, 6% and 9%, respectively. Prior to the mercury injection test, compaction specimens were fabricated from the state of extraction saturation to vacuum saturation state. The gas pressure of pores was approximate to zero in the sample of improved expansive soil for the dilatometer in mercury injection apparatus. A low-pressure mercury injection was first used to analyze the soil samples. The dilatometer was then taken out to weigh the mass on a high precision balance. A mercury pressure analysis was carried out on the soil samples until the pressure gradually decreased to atmospheric pressure. The results show that the mercury injection can be divided into the process of mercury injection and removal. When the pressure was small, there was the large equivalent diameter of pores corresponding to the pressure, where mercury can only enter the large pores in the soil sample, resulting the small cumulative amount of mercury injection. The pore diameter gradually decreased as the pressure increased, where mercury can enter the smaller pores in the soil, and thereby the accumulated amount of mercury gradually increased. With the increase in the content of coal gangue powder, the accumulative content of mercury in the improved soil significantly decreased, where the most obvious content were 6% and 9%. Large micropores decreased by 61.5% in the expansion content of macropores. There was a trend of decrease in the total volume of pores, the critical aperture porosity, average pore diameter, and pore porosity index. Pore types were transformed from the pores between the aggregates to the pores between particles, indicating an obviously enhanced effect in the addition of coal gangue to the expansive soil. In order to study the relationship between the pore fractal dimension and pore characteristics, a Menger sponge model was used to characterize the fractal dimension of pores in the improved soil, where the values varied from 2.59-2.87. A dependent law was obtained between the fractal dimension of pores and the characteristic parameters of pores in multiple linear regressions. In "F" and "t" test of regression model, it was found that the porosity and critical pore size had a significant influence on the fractal dimension of pores. The fractal dimension of pores increased in the improved expansive soil with the increase in powder content of coal gangue. The large fractal dimension indicated that there was an orderly arrangement of pores and shape characteristics in the improved soil. The fractal dimension of pores can be used to visually represent the measurement data of pore structure, further to verify the pore characteristic parameters, and pore development degree, serving as an indirect indicator of the heterogeneity and complexity of soil.