In the present study, a modified electrode has been constructed by immobilizing quercetin at the surface of a glassy carbon electrode modified with multi-wall carbon nanotubes (Q-MWCNT-GCE), and its electrochemical characteristics were studied by cyclic voltammetry. The Q-MWCNT-GCE was successfully used for N-acetyl-L-cysteine (NAC) electrocatalytic oxidation and simultaneous determination of ascorbic acid (AA), L-DOPA (DL), NAC, acetaminophen (AC) and tryptophan (Trp). The obtained results indicate that the peak potential of NAC oxidation at the Q-MWCNT-GCE surface appears at the less positive potential compared with that at the MWCNT-GCE or Q-GCE surface. The electron transfer coefficient, α, and the heterogeneous electron transfer rate constant, k′, for the oxidation of NAC at the Q-MWCNT-GCE surface were calculated to be 0.30 and 6.5 × 10⁻⁴ cm s⁻¹, respectively. Furthermore, differential pulse voltammetry (DPV) exhibits two linear dynamic ranges of 1.1–50.0 and 50.0–1000.0 μM and a detection limit of 0.44 μM for NAC determination. Also, the Q-MWCNT-GCE was used for the simultaneous determination of AA, LD, NAC, AC and Trp with DPV. Finally, the activity of the modified electrode was also investigated for the determination of AA, LD, NAC and AC in real samples and satisfactory results were obtained.