BEIJING—Chinese President Xi Jinping announced a reduction of 300,000 troops to China’s armed forces, part of a series of reforms to strengthen a military striving to assert the country’s increasingly global interests.

Mr. Xi announced the troop reduction in a speech Thursday at a military parade to commemorate the 70th anniversary of the end of World War II that featured some of China’s most up-to-date armaments, rolling through or flying over Tiananmen Square. Though primarily aimed at a domestic audience, the parade served as the latest display of muscle-flexing by Beijing that has unsettled China’s neighbors and raised concerns from Washington.

In his remarks, Mr. Xi said the “world was far from tranquil,” though he reiterated China’s commitment to peace. The Chinese military would “faithfully fulfill the sacred duty of protecting the nation’s security and people’s well-being, and carry out the noble mission of upholding world peace,” Mr. Xi said, standing on top of Tiananmen, the Gate of Heavenly Peace, which served as a reviewing stand.

In announcing the cut of 300,000 troops, Mr. Xi provided no details on how it would be carried out. A Defense Ministry statement later said the reductions would be done by the end of 2017 and would target units with outmoded equipment and noncombat personnel.

While the timing of the announcement was unexpected, the reduction fits in with an ambitious revamping meant to modernize the military, favoring the navy and air force over the army, enabling it to project power and protect China’s proliferating world-wide strategic and commercial interests.