Taking the former chemical site of Ma Steel in Anhui as the research object, extensive soil, water, and gas samples were collected and analyzed in the laboratory. Based on the experimental data, the contamination characteristics and the migration-distribution mechanisms of the site were characterized, and a risk assessment was conducted. The results reveal that contaminants are predominantly distributed in the old coking area, circulating water and wastewater treatment zone, iron-making blast furnace area, and raw material storage yard. A total of 72 organic compounds are detected in the soil samples, among which 29 exceeded screening values. The maximum exceedance multiple of the potent carcinogen benzo(a)pyrene reach 2 655 times, and its concentration is higher than that detected at coking plant sites in northwest and northern China. Heavy metals such as arsenic, lead, and cadmium also exceed standards to varying degrees. More than 40 organic compounds ware detected in water samples from the shallow perched aquifer and clay layer, but the number and concentration of contaminants decrease significantly in the confined aquifer. Thirty volatile organic compounds are identified in wellhead gas samples, and the concentration of gaseous pollutants decline notably with increasing distance from the ground surface. Both the concentration and number of contaminants in soil and water samples decrease markedly with depth. This is attributed to the tendency of contaminants to accumulate in the more permeable fill layer within 3.5 m below the surface, whereas the low permeability coefficient of the clay layer hindered downward migration. Meanwhile, the risk assessment indicated that soils within 0~4.5 m below the surface posed relatively severe carcinogenic and non-carcinogenic risks. The result systematically achieve an in-depth analysis that integrates multi-media, high precision, mechanism elucidation, and risk assessment. It not only reveals the composite contamination status of the Ma Steel site but also clarifies the migration mechanisms from the perspective of geology-contamination coupling, providing both theoretical and practical references for risk assessment and remediation of industrial legacy sites in China.