When an intense laser pulse propagates in a gaseous target, an ionization front is produced at the leading edge of the laser pulse, which moves at the speed of light. By use of a one-dimensional particle-in-cell code with the field ionization included, we simulate the interaction between the laser pulse and its ionization front. It is found that, because of the discontinunities in atomic ionization potentials for different ionization orders, a flat-top region of the electron density appears inside the ionization front at the beginning. The ionization induces blue shif in the laser frequency, which modifies the laser pulse into a step-like profile. This new pulse profile in turn shortens the flat-top region in the ionization front. Generally the ionization front evolves into several parts, corresponding to different ionization orders, with different plasma density gradients. The effects of different gas elements and initial gas densities on the produced ionization fronts have been investigated. The conditions ith regard to the laser intensity and gas species to produce steep density gradients in ionization fronts have been studied for applications in frequency up-conversion by use of counter-propagating light pulses. Finally, the spectrum evolution of the ionizing pulse is investigated and the resulting fine spectrum structure located between the fundamental and the second harmonic is analyzed.