According to China’s PBN implementation roadmap, PBN based on the instrument flight procedures will be implemented to all ATS routes after 2017. At present, Chengdu-Lasa RNP4 parallel routes is designed. This paper presents the safety assessment for the implementation of Chengdu-Lasa RNP4 parallel routes. Firstly,the methodology of collision risk of parallel routes is introduced by improved REICH model. Then, the method of lateral overlap probability is analyzed. The lateral overlap probability is divided into two parts---the core distribution and the non-core distribution. Unlike the most of previous research, the non-core distribution obey “Separated Double Exponential” distribution which is more realistic in this paper. Finally, the collision risk of Chengdu-Lasa RNP4 parallel routes is carried out. The estimate of collision risk satisfies the target level of safety value and indicates that the method is feasible.