Amorphous silicon(a:Si) films were annealed by KrF excimer laser to realize the influence of different power density and different pulse counts. The analysis of a：Si thin film microstructure and surface morphology was conducted using X-ray diffractometer(XRD) and scanning electron microscope(SEM). In the range of 1 Hz, the results show that the polycrystalline silicon structure has been achieved from amorphous silicon by excimer laser annealing when the energy density reaches about 180 mJ/cm2. When the energy density is from the energy density threshold 180 mJ/cm2 to the energy density 230 mJ/cm2, the crystallization effect gets better with the increase of the energy density. The effect of crystallization is best and the gain size is the biggest while the energy density is 230 mJ/cm2. The maximum average size of the grain reaches 60 nm and the polycrystalline silicon film grows preferentially along the crystallographic(111) orientation. The influence of pulse counts are not remarkable if the pulse counts are over 50 times.