The invention discloses a preparation method and application of a magnetic HACC (Hydroxypropyl Trimethyl Ammonium Chloride Chitosan) / oxidized multi-walled carbon nanotube adsorbent. The preparation method is characterized by comprising the steps: adding 0.2-0.6 g HACC into a solution formed by 20 mL water and 5 mL glacial acetic acid, performing ultrasonic treatment at room temperature for 2-4 h, adding 0.05-0.20 g magnetic amination ferroferric oxide and stirring for 1-2 h, adding 3 mL liquid paraffin while stirring slowly, carrying on stirring for 0.5-1 h, adding 3 mL 25% glutaraldehyde to crosslink HACC, meanwhile, adding 0.1-0.2 g oxidized multi-walled carbon nanotube for reaction at 50 DEG C for 1.5-2 h, adjusting pH within 9-10 by using 2 mol / L NaOH, keeping at 80 DEG C for 1 h, washing an obtained product until pH is 7.0 sequentially by using petroleum ether, ethanol and de-ionized water to prepare the magnetic HACC / oxidized multi-walled carbon nanotube adsorbent. The adsorbent is simple and practicable in preparation method, high in adsorption capacity of adsorbing heavy metal and high in adsorption as well as separation speed, can be utilized repeatedly, and obtains excellent effects on treating heavy metal in waste water, and no secondary pollution exists in the adsorption process.