Millions of tons of cotton textile waste are generated annually worldwide, and most of it enters the municipal solid waste (MSW) stream for landfill or incineration disposal, resulting in a significant waste of resources and environmental pollution. This article developed an innovative low-temperature pyrolysis process for producing artificial humic acid (HA) from waste cotton textiles. Subsequently, Al3+ was introduced for a secondary pyrolysis to prepare an Al-doped biochar (Al/BC) with superior adsorption properties. The experimental results show that the presence of Al3+ has an important influence on the pyrolysis process of cotton fabric and the formation and structure of Al/BC. This is the first time to synthesize low-cost adsorbent by pyrolysis and aluminum mixing process of waste cotton cloth and explains the mechanism. This increased in defects (ID/IG = 0.76 to ID/IG = 0.92), a larger specific surface area (6.47 m2/g to 566.94 m2/g), and an increase in oxygen-containing functional groups (from none to C-O-C, OC-O, etc.) when compared to the undoped Al3+ biochar. The optimum Al3+-doped biochar (Al/BC-15) prepared with HA as a precursor exhibited a superior adsorption capacity for Cr(VI) of up to 176.23 mg/g, surpassing the results reported for similar materials in the literature. The adsorption mechanism of Cr(VI) is primarily based on physical adsorption, with some chemical adsorption. At a lower pH, the Al/BC-15 surface exhibits a high positive charge (46 mV). The Al3+-O-Cr(VI) association group is formed through rapid electrostatic attraction between C-Al and Cr(VI). Due to the strong positive electronegativity of Al3+ and the negative electronegativity of C in the vicinity of Al, Cr(VI) is further reduced to Cr(III) by C-Al. Therefore, the method proposed in this paper for preparing Al-doped carbon materials from waste cotton fabric offers a new approach and potential application for the production of high-performance adsorbent materials from waste cotton fabric.