The multi-walled carbon nanotubes decorated with Co3O4 nanoparticles (Co3O4/MWCNTs) were successfully prepared by the hydrothermal method. A facile and practical electrochemical sensor based on Co3O4/MWCNTs was established for simultaneous detection of hydroquinone (HQ) and catechol (CC) by differential pulse voltammetry (DPV). The defined and separated oxidation peaks of a mixture of HQ and CC were obtained at pH 8.0, owing to the catalytic stability of Co3O4 in alkaline conditions as well as the high conductivity and excellent adsorption of MWCNTs. The proposed sensor exhibited the wide linear range from 10 to 800 mu M (HQ) and 10-700 mu M (CC), as well as the low detection limit of 5.6 mu M (HQ) and 8.5 mu M (CC). Meaningfully, in local seawater sample testing, the explored sensing platform achieved an excellent recovery for the simultaneous detection of CC (97.5-104.2%) and HQ (96.4-109.1%). According to the investigation, this is the first successful application of HQ and CC in direct seawater detection. This work demonstrated the Co3O4/MWCNTs nano-composite was a promising material for electro-catalysis and electro-analyst in the marine environment.