Abstract: In this paper, the authors have established a numerical calculation model of the HAZ of multipass welds of a new type high strength steel, according to the various st ructure of the new type high strength steel after varied heat circle. and calculated the size of local brittle zone (LBZ) in its HAZ. The cal culated results accord well with the measured ones, the maximal error is 6.69%. Based on what had been done, this paper discussed that main factors that influence LBZ. The results indict that: The weak PO sition of the HAZ of multipass weds of new type high strength steel is 0.2-0.5mm from fusion line; The influence of welding linear energy on LBZ is seasonal, but generaly speaking, the linear energy about 20kJ/cm is advantaged to reduce LBZ; And preheat is advantage to reduce LBZ, and decreasing character temperature T1 and T3 are also helpful to prevent the happening of LBZ.