The Bethe-Salpeter equation is solved in the framework of the unitary coupled-channel approximation by using the pseudoscalar meson-baryon octet interaction.The loop function of the intermediate meson and baryon is deduced in a dimensional regularization scheme,where the relativistic kinetic effect and off-shell corrections are taken into account.Based on the experimental data at the K-p threshold,the subtraction constants in the loop function are determined.The squared amplitude is suppressed strongly and only one A(1405) state is generated dynamically in the strangeness S=-1 and isospin I=0 sector.