Abstract The degradation of organic contaminants from domestic sources can lead to a reduction in soil strength and the emission of greenhouse gases such as methane (CH4) and carbon dioxide (CO2). It is crucial to explore green and low-carbon disposal methods for domestic-sources-contaminated soil. In this study, biochar was utilized to remediate domestic-sources-contaminated soil due to its stable performance and excellent adsorption and fixation of organic matter. The geotechnical tests and thermogravimetric analysis were investigated to evaluate the remediation effects of biochar dosage and particle size from environmental and engineering viewpoints. The results identified clear and effective ranges for biochar dosage and particle size in remediation of domestic-sources-contaminated soil, and biochar can simultaneously achieve environmental and engineering remediation. The optimal remediation effect was observed with a biochar dosage of 6% and a particle size of 1–3 mm. The remediation mechanisms of biochar on domestic-sources-contaminated soil primarily consisted of adsorption and aggregation, regulation of physicochemical properties, and alteration of structural. At lower biochar dosages and smaller particle sizes, adsorption and aggregation and regulation of physicochemical properties were dominant, while at higher dosages and larger particle sizes, alteration of structural became the main factor. This study preliminarily confirmed the feasibility of biochar in the synergistic environmental and engineering remediation of domestic-sources-contaminated soil, providing a basis for the development of coupling technology for carbon sequestration and remediation with biochar of such soil.