Abstract: Fast online inspection technology is a difficult problem in the manufacture of steel tube. Radiographic CT imaging technology can reconstruct the image of steel tube section and acquire its most dimension parameters, which is quite appropriate for its inspection and quality control. A scan mode with immobile ray sources and detectors is proposed to reduce the inspection time. It can obtain projection data quickly and meet the need of online inspection. Maximum a posteriori (MAP) reconstruction algorithm is modified based on the space domain and pixel value domain characters of steel tube sections in order to reconstruct its image from few projections. The results of simulation experiment indicate that the modified MAP algorithm can reconstruct the image of steel tube section from at least three projections. The precision of acquired dimension parameters meets the requirement of national standard and the technology is expected to be widely used in practice.