X-ray irradiation effect in hydrogenated amorphous silicon (a-Si:H or a-Si) solar cell has been observed. The results show that the photovoltaic characteristics decrease with the increasing of X-ray irradiation dose at first, and then tend to a steady state. By the below-gap photocurrent spectroscopy measurement, the change of defect density in a-Si induced by X-ray irradiation was studied. The results show that the defect density increase with the X-ray irradiation dose.