The invention discloses a testing apparatus and method for accurately measuring initial gas expansion energy. The apparatus realizes instant release of gas and avoids time delay; a sensor has high precision, obtained gas pressure value is more accurate, and the initial released gas expansion energy is accurately determined; coal body dead space volume and gas adsorption quantity are accurately measured, and a quantitative relation between the initial released gas expansion energy as well as a gas content and free gas amount; determination of gas adsorption quantity and initial gas expansion energy is realized, the purposes for once adsorption of a sample and multiple parameter determination are realized, test efficiency is improved, and test time is saved.