The construction of new power system is an important means to achieve the goals of " carbon emission peak and carbon neutrality ", and a large number of new energy access with strong randomness, volatility and intermittently makes the safe and stable operation of the future power system face severe challenges. In line with regional status advantages and promoting economic development, large users usually have complex load characteristics and high power supply reliability. In order to reduce the influence of large users' settlement on the stable operation of the power grid and other constraints, this paper proposes to take large users and the power grid as a whole. In the DigSILENT/PowerFactory simulation software, a three-state Markov fault model was established between the large-user substation and the upstream power grid contact line, the bus bar in the station, the main transformer, the protection device, etc., and the corresponding reliability parameters were set. Three reliability evaluation indexes are proposed: annual failure frequency, annual failure duration and average power supply availability. For a large user in a coastal city, different grid access schemes are proposed, and the reliability indexes of the schemes are evaluated in the year of operation and five years after the large-scale user is put into operation. The results show that the reliability index evaluation based on the simulation results of the three-state Markov fault model can provide technical support for the power grid company to make decisions on the schemes of large user access to the power system.