Firstly, the attosecond pulse generation of CO molecule[12] driven by a 800nm2000hm combined laser field is theoretically studied based in the strong field approximation[3],where the effect of stark shift [4]is also taken into account.The relative carrier envelope phases of the combined field can be used to optimize the laser field.The results show that the cut-off of harmonics is extended and the supercontinum of harmonics is improved because of the ionization suppression of asymptomatic molecule in a certain direction, which reduce the quantum interference in the propagation.The isolated attosecond pulse could be obtained by superimposing the plateau region of harmonics spectrum, and the width of the shortest single attosecond pulse is 98as.Secondly, the ionization of atoms exposed to an intense laser field has been studied by three different methods, i.e., the numerical solution of the single-activeelectron approximation based time-dependent Schr(o)dinger equation (SAE-TDSE)[5], the Perelomov-Popov-Terentev model (PPT), and the Ammosov-Delone-Krainov model (ADK).We also investigated the ionization of several linear molecules in a strong laser field with the molecular ADK (MO-ADK)[6] and the molecular PPT (MO-PPT) model[7], which shows that the ionization probability and signal obtained from the PPT and the MO-PPT model fits well with the SAE-TDSE resuh and experimental datafrom the multiphoton to tunneling ionization regimes.However, both the ADK and MO-ADK models underestimate the ionization probabilities and signals in the multiphoton regime.