The invention discloses a method for determining a movable fluid space of a rock core. The method comprises the following steps: step 1), drilling a rock core column with the diameter of 2.5cm and the length of no less than 3.0cm from a rock core; cutting a section; after washing with oil, obtaining porosity, gas permeability and pore volume Vp of the rock core; 2) testing a water movable space Vw of the rock core; 3) testing an oil movable space Vo of the rock core; 4) calculating movable water and oil saturation degrees of the rock core respectively through the Vw and the Vo. The invention provides a brand-new method for testing the movable space of the rock core; oil and water can be used for displacing and the volumes of displaced fluid under different pressure are recorded at the same time; the movable fluid space of the rock core under the pressure can be calculated through metering the volume of flowing-out fluid; the method is simple and easy to operate and the evaluation is more direct.