Effect of slow neutron irradiation of low fluence on bismuth-based superconductors is studied. It's shown through experiments that (1) both critical current density Jc and zero-resistance temperature Tc0 are increased, though to different extents, after the Bi-based superconductors were irradiated with slow neutrons to suitable fluence and (2) slow neutrons of low fluence and fast neutrons of high fluence have similar irradiation effects on Bi-based superconductors.