A method for manufacturing graphite oxide, the method comprising: preparing graphite slurry by mixing graphite and a sulfuric acid at a temperature in a range of from 0 °C to 60 °C; mixing the graphite slurry with an oxidizer selected from the group consisting of a permanganic acid, permanganate, and a permanganate sulfuric acid solution by supplying the oxidizer by stages into sequencing batch oxidizer mixers connected to one another in multiple stages, wherein the oxidizer is supplied to the sequencing batch oxidizer mixers sequentially by a fixed-quantity pump connected thereto; making an intermediate reaction by pressing a reaction mixture mixed with the oxidizer into a tube type reactor having an inner diameter of from 5 mm to 50 mm, and a length in a range of from 10 m to 1000 m, and equipped within a thermostat bath to be applied with ultrasonic waves and keeping the reaction mixture for a residence time in a range of from 0.2 hours to 5 hours, wherein the tube type reactor is maintained at a temperature in a range of from 5 °C to 54 °C, wherein 50 % to 70 % of the total oxidation reaction of the graphite occurs in the tube type reactor, wherein, preferably, the rest of the oxidation reaction additionally occurs in a continuous stirred tank reactor; making an additional reaction by supplying a reaction mixture of the intermediate reaction into the continuous stirred tank reactor at a temperature of from 8 °C to 30 °C; mixing a reaction mixture of the additional reaction with water added; adding a hydrogen peroxide aqueous solution to the reaction mixture mixed with the water and mixing them to terminate an oxidation reaction; washing and separating graphite oxide from the reaction mixture of the terminated oxidation reaction; and drying the separated graphite oxide, wherein a carbon/oxygen ratio of the graphite oxide is in a range of from 1/1 to 5/1 according to an elemental analyzer and the graphite oxide has a maximum peak in 2θ of from 12° to 14° according to an XRD analysis. preparing graphite slurry by mixing graphite and a sulfuric acid at a temperature in a range of from 0 °C to 60 °C; mixing the graphite slurry with an oxidizer selected from the group consisting of a permanganic acid, permanganate, and a permanganate sulfuric acid solution by supplying the oxidizer by stages into sequencing batch oxidizer mixers connected to one another in multiple stages, wherein the oxidizer is supplied to the sequencing batch oxidizer mixers sequentially by a fixed-quantity pump connected thereto; making an intermediate reaction by pressing a reaction mixture mixed with the oxidizer into a tube type reactor having an inner diameter of from 5 mm to 50 mm, and a length in a range of from 10 m to 1000 m, and equipped within a thermostat bath to be applied with ultrasonic waves and keeping the reaction mixture for a residence time in a range of from 0.2 hours to 5 hours, wherein the tube type reactor is maintained at a temperature in a range of from 5 °C to 54 °C, wherein 50 % to 70 % of the total oxidation reaction of the graphite occurs in the tube type reactor, wherein, preferably, the rest of the oxidation reaction additionally occurs in a continuous stirred tank reactor; making an additional reaction by supplying a reaction mixture of the intermediate reaction into the continuous stirred tank reactor at a temperature of from 8 °C to 30 °C; mixing a reaction mixture of the additional reaction with water added; adding a hydrogen peroxide aqueous solution to the reaction mixture mixed with the water and mixing them to terminate an oxidation reaction; washing and separating graphite oxide from the reaction mixture of the terminated oxidation reaction; and drying the separated graphite oxide, wherein a carbon/oxygen ratio of the graphite oxide is in a range of from 1/1 to 5/1 according to an elemental analyzer and the graphite oxide has a maximum peak in 2θ of from 12° to 14° according to an XRD analysis.