Fungal laccases are typically unstable at high pH and temperature conditions, which limit their application in the decolorization of textile wastewater. By contrast, the highly stable bacterial laccases can function within a wider pH range and at high temperatures, thus have significant potential in treatment for textile wastewater. In our previous work, a thermo-alkali-stable CotA-laccase gene was cloned from Bacillus pumilus W3 and overexpressed in Escherichia coli. In this study, the robust CotA-laccase achieved efficient secretory expression in Bacillus subtilis WB600 by screening a suitable signal peptide. A maximum CotA-laccase yield of 373.1 U/mL was obtained at optimum culture conditions in a 3-L fermentor. Furthermore, the decolorization and detoxification of textile industry effluent by the purified recombinant CotA-laccase in the presence and absence of redox mediators were investigated. Among the potential mediators that enhanced effluent decolorization, acetosyringone (ACS) was the most effective. The toxicity of the CotA-laccase-ACS-treated effluent was greatly reduced compared with that of the crude effluent. To the best of our knowledge, this study is the first to report on the heterologous expression of CotA-laccase in B. subtilis. The recombinant strain B. subtilis WB600-5 has a great potential in the industrial production of this bacterial enzyme, and the CotA-laccase-ACS system is a promising candidate for the biological treatment of industrial textile effluents.