The invention discloses a preparation device of a plasma-modified glass fiber. The preparation device comprises a pulse power source device, a discharge device and a measuring device, wherein the pulse power source device comprises a pulse power source and a voltage regulator connected with the pulse power source; the discharge device comprises a high-voltage electrode and a ground electrode; the lower surface of the high-voltage electrode is provided with a unilateral countercheck medium; the measuring device comprises an oscillometer. The preparation device can be used for solving the problem that the modified glass fiber prepared by an existing modification method is low in heat resistance and the composite material prepared by the modified glass fiber can not adapt to use environments with relatively high temperature. The invention also discloses a method for peparing the plasma-modified glass fiber. The method comprises the following steps: step 1, preprocessing the glass fiber; step 2, performing plasma modification on the preprocessed glass fiber by use of the preparation device of the plasma-modified glass fiber. The invention also discloses a composite material prepared by use of the plasma-modified glass fiber, and the mass ratio of the plasma-modified glass fiber and epoxy resin is 30 to (100-110).