He promised to "lead all the priests seminarians and nuns of this diocese in adhering to the nation's constitution, maintaining national unification and social stability."
The Vatican, whose relations with China have been strained by Beijing's insistence that Catholic church affairs are a state matter, has made no statement about whether his appointment had the formal approval of the pope.
But officials in Rome have indicated the appointment is viewed favorably and would help relations.
China severed ties with the Vatican in 1951 and demanded Chinese Catholics also cut links after the Holy See recognised Taiwan instead of Beijing
All religion was outlawed in China in the 60s and 70s but the church now has about 12 million followers
However, many Catholics worship in underground churches instead of state-approved ones, reflecting a contest for authority between Beijing and the Vatican
The appointment of bishops has been a regular source of conflict, with China's state-run church insisting it has the right to do so and the Vatican excommunicating those deemed illegally ordained
Recently tensions have eased as the Vatican seeks greater access to China where religious belief is growing while Beijing seeks greater global legitimacy
Since taking office in 2005, Pope Benedict XVI has made several attempts to reach out to Beijing.
Although he has maintained that the Vatican should choose bishops - with possible government consultation - and urged China to permit more religious freedom, the pontiff has also called for healing between divided believers.
He has also written to China's leaders to urge restoration of diplomatic ties with the Vatican.
When Li's appointment was announced in July, Cardinal Tarcisio Bertone, the Vatican secretary of state, welcomed the decision as a "positive sign" and said Li was "well-suited" for the role.
Liu Bainian, the vice-chairman of the Chinese Patriotic Catholic Association, the government body that oversees the church, said he "was not too clear" about the Vatican's stance towards Li.
But he added: "We know that the pope loves China."
The 42-year-old replaces Bishop Fu Tieshan, a Communist party supporter and hardliner towards the Vatican.
Observers say Fu's death in April provided an opportunity for rapprochement between China's state-controlled church and Rome after years of often frosty relations.