State media reports say the flood struck Xiaoyuhecun Valley on Monday afternoon amid the heaviest rains recorded in the area in 30 years.
A team of about 200 police officers, firefighters and paramilitary troops searched the area overnight, recovering eight bodies by Tuesday morning, Xinhua reported.
A former imperial Chinese capital, Xi'an lies in a greenbelt along the ancient Silk Road surrounded by the mountainous, mostly arid, province of Shaanxi.
Many Chinese scenic sites suffer from chronic mismanagement, poor crowd control and other safety issues.
Emergency services in Xi'an either didn't answer calls or said they had no updated information.