Unequal-thickness tailor welded blanks （TWB）are widely used in automotive body manufacturing，equal strength design is realized by using this method. TWB ensured the body’s strength and safety，and achieved a lightweight and energy saving cars. Undercut is one of the common defects of unequal-thickness tailor welded blanks which affecting its tensile strength seriously. Based on technological features of tailored laser welding，the causes and solutions of undercut are studied， the relationships between line energy，mismatch，fine cut error and undercut are made，and the minimum melting width of the thinner blanks is obtained. The research results can provide theoretical support for optimizing tailored laser welding process parameters.