The laser beam spot evolution and electron density in the guiding range are evaluated with the burn pattern and the measured conductivity . the above results will be useful in ultrashort laser pulse amplification and future research in ultrashort intense laser propagation in plasmas and ionizing gases 以上研究结果对于超短脉冲激光放大技术研究以及超短超强脉冲激光在低密度等离子体和空气中长距离传输和聚焦研究等具有重要的实用参考价值。