The reasons affecting on longitudinal surface cracks of wide slab are analyzed. Therefore, we can take preventive measures to improve steel quality based on the production practice. It is concluded that surface longitudinal cracks appear easily in the microalloy steel containing Nb and the steel in high sulfur content and whose S ratio at manganese are less than 30 . The steel in peritectic area whose carbon content is in 0.12%~0.17% are also easy to produce surface longitudinal cracks. In addition, we further analysis the longitudinal surface cracks on wide slab from a series of key facors, such as casting speed、flux、 immersion depth of nozzle and level fluctuation in mold.