In this paper, we analysed the diamagnetization flux of samples with different lengths by calculating the distribution of the magnetic field produced by magnetization current. A simple and clear correction factor which was determined by the coil size and samples' lengths was obtained. An accurate calculation method for absolute magnetization of samples with small cross section was presented. Through the measurement of the susceptibility on BiPbSrCaCuO superconductors with different lengths, this method is shown to be valid.