Abstract: Time response of GaN Schottky detector with a large area to X-ray was studied. Using a Fe-doped GaN high resistive film to make the detector, the time response under different bias was tested. For the measured results, a theoretical model of time response of GaN Schottky detector to X-ray irradiation was proposed, and its internal mechanism was studied with a very good fitting results. It is found, due to the presence of high resistivity layer, the GaN Schottky detector can have a high signal to noise ratio of about 80 at reverse bias of 200 V, even in the possible effects of light quenching.