Xi became China's most powerful leader in generations last week when he was re-elected for another five years at the helm of the world's most populous nation, in a break with long-standing political precedent.
The National People's Congress (NPC) - China's parliament - also appointed a key Xi ally, Li Qiang, as the new premier.
Xi, 69, on Monday thanked the thousands of delegates at Beijing's Great Hall of the People for giving him a third term, vowing to "take the needs of the country as my mission, and the interests of the people as my yardstick".
"Security is the bedrock of development, while stability is a prerequisite for prosperity," Xi told the assembled delegates at the NPC's closing session.
"We must fully promote the modernisation of national defence and the armed forces, and build the people's armed forces into a Great Wall of steel that effectively safeguards national sovereignty, security and development interests."
Xi also called for consolidated stability in once-restive Hong Kong and unification with the self-ruled island of Taiwan, which Beijing views as part of its own territory.
"The trust of the people is the greatest driving force pushing me forward, and also a heavy responsibility on my shoulders," he said.
"The great rejuvenation of the Chinese nation has entered an irreversible historical process."