The invention discloses a large-area CMOS image sensor for detecting X-rays directly. The large-area CMOS image sensor comprises a PCB (printed circuit board) and a plurality of image sensor chips, wherein each image sensor chip comprises a P-type semiconductor silicon substrate, and pixels, timing sequence generation circuits and column signal processing and buffer output circuits are arranged on the P-type semiconductor silicon substrates; the image sensor chips are distributed on the PCB in an array manner; and the distance between the pixels in corresponding positions on edges of the adjacent image sensor chips is 1 time or 1.5 times that between the two adjacent pixels on the same image sensor chip. The image sensor is simple in structure, low in cost, convenient to manufacture and capable of conducting large-area X-ray detection.