Abstract: The basic condition required for achieving central ignition is producing a hot spot with 10 keV temperature and 0.3 g/cm2 surface density. Growth of hydrodynamic instability during deceleration phase will destroy the symmetric-drive , reduce the volume of central hot spot and make a harmful effect on ignition. Based on the LARED-S code, considering the thermonuclear reaction and α-particle heating, a numerical study of direct-drive ICF in spherical geometry is made. One-dimensional results agree well with the NIF ignition target designs, and show that the α-particle heating plays an important role in marginal ignition. Two-dimensional results show that the growth of hydrodynamic instability during deceleration phase makes a harmful effect on ignition.