As the first type of practical fountain codes, LT codes are used in various applications. Although the performance of an LT code is determined by the parameters of the Robust Soliton Distribution RSD, this relationship remains largely unclear. In this paper, we analytically study the relationship between the mean degree value of the RSD and the parameters including the spike position and δ in RSD. Then, with obtained properties and simulation results, we further investigate the impact of these parameters on the performance of LT codes.