Carboxymethyl cellulose (CMC) was introduced as a depressant in reverse flotation separation of quartz from magnesite. The flotation behavior and surface properties of magnesite and quartz exposed to CMC were studied by zeta potential tests, atomic force microscopy imaging and contact angle measurements. The addition of CMC as the depressant in reverse flotation using dodecylamine (DDA) as the collector exhibited a selectively depressive performance towards magnesite and achieved an improved recovery of magnesite. The study of surface properties demonstrated that CMC and DDA exhibited different adsorption strengths on the surface of magnesite and quartz. It was found that the adsorption of CMC on magnesite surface was stronger than that of DDA, which hindered the subsequent adsorption of DDA on magnesite surface. On the contrary, the quartz surface was strongly adsorbed by DDA instead of CMC, which proved that the addition of CMC did not influence the flotation of quartz.