The disclosure provides a method for preparing an amphiphilic lignin nanomaterial based on pulping black liquor, an amphiphilic lignin nanomaterial, and an oil sludge detergent. In the disclosure, physical treatments such as ball milling and high-pressure jet homogenization treatment are conducted on alkali lignin at the early stage to adjust the molecular weight and size of alkali lignin and thus to give alkali lignin nanoparticles with uniform particle sizes; and on this basis, a chemical treatment such as alkylation grafting modification is conducted to give amphiphilic lignin nanoparticles with both hydrophilicity and lipophilicity. Due to the nano-size effect, the amphiphilic lignin nanomaterial has a significantly-increased specific surface area (SSA) and effectively-improved surface properties, which can reduce the oil-water interfacial tension, and emulsify the crude oil and peel off the crude oil from the surface of rock particles, so as to achieve the purpose of oil-solid separation.