In his message, Xi said he was shocked by the news of the crash, which resulted in heavy casualties.
On behalf of the Chinese government and the Chinese people, and also in his own name, Xi expressed his sympathies to the injured and conveyed his deepest condolences to the bereaved families.
Chinese Premier Li Keqiang also sent a message to his Algerian counterpart, Ahmed Ouyahia, to express his condolences over the accident.
The military plane crashed Wednesday morning in Algeria's Blida Province, 30 km southwest of the capital Algiers, killing all 257 people on board, including 10 crew members.
It is confirmed by Algeria's Defense Ministry as the worst air disaster in the country's history.