A detection device for a CT system comprises a low-energy detector assembly; and a high-energy detector assembly disposed under the low-energy detector assembly. The high-energy detector assembly comprises: a plurality of rows of high-energy detectors arranged at predetermined intervals. With the detection device, detectors and data acquisition units are greatly reduced. A high-resolution three-dimensional CT image is acquired while high-accuracy hazardous article alarm is achieved. The cost of manufacture of the system is greatly decreased while high system performance is ensured.