The closed-door summit, which takes place once every five years, determines who rules China and the country's direction for the next term.
Mr Xi, who became the leader in 2012, has been consolidating power and is expected to remain as party chief.
The congress, which also decides on a roadmap for China for the next five years, is expected to finish next week.
Shortly after the congress ends, the party is expected to unveil the new members of China's top decision-making body, the Politburo Standing Committee, who will steer the country.
Mr Xi began his speech on Wednesday listing China's achievements during his term, and said socialism with Chinese characteristics had entered "a new era".
He called on party members to "always share our fate with the people, always keep the better life for the people in mind".