Attapulgite-modified loess has been shown to improve impermeability, offering a viable approach for engineering applications; however, cracking often occurs after repeated wetting–drying cycles. In this study, hexadecyltrimethylammonium bromide (HDTMA) was introduced to investigate its effectiveness in enhancing the wetting–drying crack resistance and hydraulic stability of attapulgite-modified loess for landfill liner applications. A series of wetting–drying cycle tests, direct shear, consolidation, and permeability tests, along with scanning electron microscopy (SEM) analyses, were conducted to evaluate the modified loess systematically. According to the experimental results, when the attapulgite and HDTMA contents are 10% attapulgite + 2% HDTMA, the permeability coefficient of the modified loess remains below 1 × 10−7 cm/s after five wetting–drying cycles, indicating excellent impermeability stability and crack resistance. In addition, the composite modification effectively enhances the mechanical properties and structural stability of loess. These findings suggest that attapulgite-HDTMA composite modification is a feasible and promising strategy for enhancing the suitability of loess as a landfill liner material, demonstrating considerable potential for practical engineering applications. Nevertheless, further investigations are still required, particularly regarding long-term durability, consolidated drained shear behavior, field-scale performance, and the effects of a wider range of modifier concentrations.