A type of cationic decolourising agent was prepared and applied to printing and dyeing wastewater treatment. The effects of the concentration of the decolourising agent, the pH value and stirring speed on decolourisation efficiency were studied. The results show that the optimum decolourisation process of the decolourising agent is determined as follows: at room temperature, a dosage of the decolourising agent of 7 mg/l, pH =7.0, and a stirring speed of 150 rpm. The decolourising agent applied to decolourise a direct dye, strong acid dye, weak acid dye and reactive dye in simulated wastewater showed high decolourisation efficiency in all cases. The range of decolourisation efficiency from the highest to lowest is as follows: direct dye, weak acid dye, strong acid dye, and reactive dye.