The paper expounds how the 420mm super thick low alloy structural steel Q345B steel plate with mechanical properties and flaw detection guaranteed was developed in Nangyang Hanye Special Steel Co.，Ltd.by appropriate composition design，mold casting，ingot heating，3800mm mill rolling and heat treatment, on the production line of converter—mold casting—rolling—normalizing. After heat treatment, the steel plate was inspected. The results showed that the yield strength was 305 to 350 MPa ,the tensile strength 500 to 555 MPa , percentage elongation 23% to 28% and the longitudinal impact energy at 20℃was 109 to 287 J. The properties satisfied the demand of the specification for 250 thick Q345B steel.