Abstract: With the help of radiation hydrodynamic code Flash, we simulated the ablation process of a small focal-spot nanosecond laser irradiation on an aluminum planar target. The irradiating laser has a focal spot radius of 25 m and maximum intensity of about 1013 W/cm2 with wavelength of 532 nm. In our simulations, it is observed that electron density in the laser channel is relatively lower than the ambient one in some coronal region. This phenomenon is found to be sensitive to the electron heat conduction model. By comparing simulation results with our experimental electron density profiles, flux limit factor of 0.08 is then determined in our simulation. The annular jet also occurs in our simulations and experiments. The mechanism of annular jet formation is analyzed and discussed, which is mainly related to the radiative cooling.