Traditional cylindrical back-projection algorithm depends on the cylindrical model and calibration accuracy. While object distance changes, a second calibration needed. A new flexible algorithm is proposed to simplify the process and improve the scene adaptability. According to perspective projection theory, pixel equivalent is distributed by conics. The algorithm gets cylindrical imaging characteristics to realize cylindrical back-projection by means of fitting conics in the horizontal and vertical directions separately. It can adapt to the changes of imaging characteristics through amendment of conics coefficient without calibrate again, when the target object moves around relative to the camera. Experiment results show that the proposed algorithm accuracy is about normal for traditional way and remains unchanged while the target object moves around within the scope of a certain scale. The proposed algorithm meets image transformation requirements with calibration only once when the target object moves around on a small scale and scene adaptability is improved.