Millions of Chinese died in the war, and the government believes Japan has never properly atoned for its wartime actions.
Speaking at a medal ceremony for veterans and their descendants, Xi recalled the brutality the "fiendish" Japanese invaders inflicted upon China.
"The Japanese militarist invaders were extremely bloody and cruel, who treated the Chinese people with unprecedented brutality, and tried to use massacres and death to get the Chinese people to yield," Xi was quoted as saying by the official Xinhua news agency.
"In the face of the butchers' knives of the invaders, the Chinese people used their flesh and blood to build a new Great Wall."
China remains hostile toward Japan when it comes to WWII, particularly over events like the 1937 Nanjing Massacre, in which it says Japanese troops killed 300,000 people. An Allied tribunal put the death toll at 142,000, but some Japanese politicians and scholars deny a massacre took place at all.
Xi's unusually strong remarks came on the day before Beijing holds a military parade marking the 70th anniversary of the end of World War II. The parade will feature some 12,000 troops, along with military vehicles.
Most Western leaders have decided not to attend due to concerns about the message the parade sends at a time when China is jangling nerves around the region with its growing military assertiveness, for example in the disputed South China Sea, Reuters reported.
Japanese Prime Minister Shinzo Abe is among those not attending.
Xi said that victory in the war was a "miracle" given the difficulties China faced, but that the lesson has never been forgotten, Reuters reported.
"In recent times, in the face of incursions again and again by strong foes, the Chinese people have never yielded, continuing to join forces, taking up positions as those in front fall, tenaciously struggling, swearing to fight to the end," he said.