Being a fuel-saving technique, aerobraking has become increasingly popular in aeronautic industry and planetary exploration. It has been used in Mars missions by NASA.To realize aerobraking on Neptune, we started with the general situation, and obtained equations that can be universally applied based on fluid mechanics, celestial mechanics and kinematic. After captured by Neptune, the orbit of the spacecraft degenerates from the initial elliptical orbit to the final circular orbit due to many aerobraking passes through Neptune atmosphere. Based on the analysis about the general situation and reasonable assumptions, it was possible to calculate and describe the orbit after each time of aerobraking. Moreover, the relation between total durations and incidence angles was discussed based on the sensitivity and was fitted as a 6-order polynomial.