The metal magnetic memory (MMM) technology, a new nondestructive testing (NDT), has potentials of quantitative evaluation on welded joint defects. This paper investigates MMM signal quantitative characteristic of welded joint defects. Steel Q235 welded joint specimens, prefabricated with nonpenetration and slag inclusion, have been scanned by MMM testing instrument at 0kN, 150kN and 230kN tension loads on tensile machinery. For the nonpenetration specimens, MMM peak amplitude ΔHp and the peak width D adds with the increasing of tensile load, but gradient K varies a little. As for the slag inclusion specimens, the ΔHp and K enhances along with tensile load increasing, but the D varies a little. The X-ray testing results at 0kN, 150kN and 230kN tension loads show the validity of MMM signal characteristic. This means the feasibility of the weld qualitative and quantitative evaluation based on MMM method.