Hefei Advanced Light Facility (HALF) is aimed to be a world-class diffraction limited storage ring (DLSR) in the soft X-ray & VUV regime. The characteristics and law of beam emittance evolution due to intra-beam scattering (IBS) in HALF storage ring was studied based on equations of equilibrium emittance. The results show that, it is necessary to comprehensively optimize the key parameters to obtain excellent beam performance in physical design of middle to low-energy DLSR. Applying the key parameters optimization strategy and taking methods to release the emittance growth due to IBS into consideration, the new version of HALF lattice can meet the needs of soft X-ray diffraction limit.