Both temporal and spatial characteristics of air plasma induced by tightly-focused laser pulse with central wavelength of 800 nm and pulse width of 50 fs are numerically simulated on the basis of a three-dimensional spatial ionization model with rotational symmetry,and the dynamic evolution of refractive index in whole plasma area is revealed.The simulation results clearly show that,when the laser pulse begins to focus,the ionization of air firstly appears in the region near the focal point.The initially io...