The invention relates to an ocean dissolved inorganic carbon in-situ measurement instrument, which relates to ocean monitoring, and is provided with a four-way valve, an injection pump, an acidification reactor, a CO2 membrane separation and absorbing device, a CO2 selective permeable membrane capillary tube, a C4D detector, a detector capillary tube, a signal generator, an acid liquid constant flow pump, an absorption liquid constant flow pump, a microprocessor, a computer, a standard sample storage tank, an acid liquid storage tank, an absorption liquid storage tank, a pure water circulator, a standard sample diluter, an acid liquid diluter and an absorption liquid diluter, wherein a seawater sample inlet, the standard sample diluter and an outlet of the pure water circulator are connected with the four-way valve and are then connected with the acidification reactor through the injection pump; the acidification reactor is provided with an acid dosing port; an outlet of the acidification reactor is connected with a sample inlet of the CO2 membrane separation and absorbing device; at least one CO2 selective permeable membrane capillary tube is arranged in the CO2 membrane separation and absorbing device; an absorption liquid outlet of the capillary tube is connected with the C4D detector; waste liquid of an outlet of the C4D detector is connected with the pure water circulator.