The adsorption of sulfamethazine (SMZ) by pristine and hydroxylated multi-walled carbon nanotubes (P-MWCNTs, H-MWCNTs) was studied under varied pH, ionic strength, cations and anions in solution. The results suggest that the SMZ adsorption onto MWCNTs can be depicted well by the pseudo-second-order and Langmuir models. The adsorption of SMZ onto MWCNTs was ionic strength and pH dependent, which indicated hydrophobic and electrostatic interactions, may be the main adsorption mechanisms. The presence of cations at 0.5 mM showed different effects on SMZ adsorption. Cu²⁺ slightly decreased SMZ adsorption by 10% to 20% at solution pH of 2.3 and 4.9, due to the competing effect of Cu²⁺ with SMZ⁺ and SMZ^±. But Cu²⁺ increased SMZ⁻ adsorption by 20% to 60% at solution pH of 7.4 and 10.0, due to the facilitating effect of the complex formation of Cu²⁺–SMZ. Al³⁺ promoted the SMZ adsorption onto P-MWCNTs, which can be attributed to the facilitating effect of Al³⁺ through a metal bridge, while inhibiting the SMZ adsorption by H-MWCNTs, which can be ascribed to the competing effect between Al³⁺ and SMZ for the negatively charged functional groups and the shielding effect of adsorbed Al³⁺ with a larger hydration radius at pH of 2.3, 4.9 and 7.4. SMZ adsorption was slightly decreased with the addition of 0.5 mM anions (Cl⁻, CO₃ ²⁻, SO₄ ²⁻, PO₄ ³⁻) due to the increase in density of negative charge on the MWCNTs' surface. The μ-FTIR results showed that π–π interaction may also play an important role in the adsorption process.