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Biomass ash (BA) is rich in silica, aluminum, potassium, phosphorus, and other elements, and it can be used for environmental remediation. However, there is a limitation in using BA for the modification with high adsorption capacities of heavy metals (HMs) through a simple and low-cost method. In this study, modified biomass ash (MBA) was prepared through one-step single alkali (KOH) modification, and its influence on the adsorption and immobilization of HMs was investigated in water and soil. BET analysis indicated that the specific surface area of MBA was increased by 3.26 times compared to BA. Compared to BA, the Si–O–Si/Al diffraction peaks of MBA were intensified, and the intensity of sylvite and gypsum was decreased based on the FTIR and XRD analysis. The maximum adsorption capacities of Cu2+, Cd2+, and Pb2+for MBA were 2.35, 1.64, and 0.30 times higher than those of BA, respectively. XPS, FTIR, and XRD analysis showed that MBA adsorbed the HMs through physical adsorption, surface complexation, ion exchange, and precipitation. The application of 0.1–0.6 % MBA increased soil pH by 0.07–0.56 units compared to BA. MBA significantly decreased the exchangeable HMs concentrations, and converted them into stable fractions. This study provided a novel amendment with high adsorption capacities and immobilization effects for HMs, which can be utilized for remediating the contaminated soils.