Abstract: The chemical composition, rolling and cooling process, stress-strain curve and microstructure of the Q345GJC high-rise building steel（the high-rise building steel with yield strength of 345MPa） were examined and analyzed. The results indicate that, the chemical composition, rolling and cooling process design of the Q345GJC high-rise building steel are reasonable, but there are a lot of unqualified steel plates in tensile test, the stress-strain curves have no yield platform. There is a layer of bainite with different thickness on the upper and lower surface of the steel plate, and there is a different degree of bainite segregation band in the heart, which leads to high yield strength and unqualified elongation. Based on this, the rescue measures of natural aging for 2 days or more after the specimen processing are put forward. Thus, the yield strength of the steel plate is reduced, the elongation of the steel plate is increased, and the tensile test of the steel plate is qualified. The qualified rate of tensile test of the 50mm specification Q345GJC steel plate is greatly improved by using above method. The qualified rate increases from 77.3% in 2016 to 96.2% in 2017.