The invention relates to an electrochemical device for wastewater treatment and a method for treating wastewater by using the same. The electrochemical device consists of a power supply, a primary electrolytic bath B, an electrolytic bath A, an electrolytic bath C, a cathode, a permeable isolation material part, an anode, a raw water tank, a purified water collector and a concentrated solution collecting tank. The electrochemical device can simultaneously remove common negative ions such as nitrate radicals, phosphate radicals, chloride ions, fluorine ions and the like and organic substances, does not need to add any agent, can avoid second pollution to the effluent, is convenient to operate, and has high treatment efficiency and low energy consumption.