Due to the great difference of mechanical properties between the preliminary and recheck tests of Q345T steel sheet pile, the reason was analyz by the means of chemical analysis, microstructure observation and mechanical performance test. The results show that the banded structure, Widmanstatten structure and banded inclusions are direct reasons for causing low tensile strength of steel. By tracing the whole production process of Q345T steel sheet pile, the inadequate removal of gas and inclusion, the aggregate segregation of impurities at late stage of slab casting, and the uneven heating temperature control are the root causes of unqualified microstructure and performances of the steel.