The mechanical properties and microstructure of the high strength low alloy HC340LA were obtained with different strain rate. The research shows that the better plasticity, higher tensile strength and yield ratio can be found in high speed tensile state than in quasi-static tension. The plasticity and tensile strength decrease with the increasing of the strain rate during the high speed tension. With the increasing of the strain rate, the grain size of the ferrite decreases and its distribution is uneven, and the grain boundaries increases. The HSLA steel HC340LA submit to obvious Ductile Frecture mechanism. The size of the dimples is more uniform, bigger and deeper with the strain rate 50 s-1 than with the strain rate 200 s-1. Therefore, the higher strain rate with over strain rate 50 s-1 has less Superscript textcontribution to the improvement of plasticity of the HSLA steel HC340LA.