The paper presents simulation results of hydrogen-air mixture ignition by an ultraviolet followed by a near-infrared laser pulse. We used the three-temperature plasma model in a combination with the beam propagation method. Effect of plasma defocusing on the ignition kernel development is reported. It is shown that the plasma generated by the pre-ionizing pulse affects the position of the focal region of the second heating pulse with resulting changes in the distribution of the laser energy absorption, electron and radical production and ignition kernel dynamics. For the case with the defocusing effect we observed a shift of the peaks of translational and vibrational temperatures towards the direction of the laser pulse and two peaks on the axial profile of the electron temperature, reflecting local changes both in Joule heating and electron energy losses.