A detector for measuring scanning ion beams in radiation therapy sequentially includes a first high voltage electrode, a first spacing member, and a segmented electrode. The first spacing member is connected to the first high voltage electrode and the segmented electrode to form a first ionization cavity. The first ionization cavity is formed with a plurality of first reading electrodes and a plurality of second reading electrodes therein. A second spacing member and a second high voltage electrode are further sequentially disposed. The second spacing member is connected to the second high voltage electrode and the segmented electrode to form a second ionization cavity. The first reading electrodes and the second reading electrodes are respectively formed in the first ionization cavity and the second ionization cavity. With the first reading electrodes and the second reading electrodes in different directions, highly accurate space resolution, space dosage and scanning speed are achieved.