Cone-beam helical Computed Tomography(CT) can scan a long object, but the conventional scanning mode of X-ray CT imaging is limited by the width of planar detector when a large object is inspected. As an alternative, a scanning and reconstructing algorithm is presented based on the Field of View(FOV) for half-cover scanning to extend the diameter of the FOV for the cone-beam helical CT. In scanning, the gantry is translated horizontally, whose direction is vertical to the central ray and rotary axis. Then the conventional helical scanning is used to obtain the projection data needed. During reconstruction, the extended FDK algorithm can provide the same efficiency as the conventional algorithm without resetting the projection data. Experimental results indicate that the half-cover scanning can extend the FOV up to 1.86 times that of the conventional method, and the image quality reconstructed by improved algorithm with a small planar detector is almost the same as that of the conventional algorithm with a large planar detector. The reconstruction time has reduced by 376.66 s due to the decrease of projections. These results show that the half-cover scanning of helical cone-beam CT can extend the diameter of the FOV, and can offer a higher calculation efficiency and low reconstruction time.