The International Ski Federation says world junior bronze medallist Wallace is in a "light coma but stable" after being flown to the University Hospital Zurich by helicopter.
The Swiss ski team, who Wallace has been training with, say the 22-year-old is showing "positive signs".
A detailed prognosis "will become clear in five or six days", they added.
"The whole situation is hard," said Christoph Perreten, the head of Freestyle Swiss Ski. "Our thoughts and our strength now belong to Lloyd and his family."
The crash reportedly happened when Wallace fell on a run-up while at a training camp in Mettmenstetten on Thursday.
Wallace trains full-time with the Swiss aerial skiing team and has been using a crowdfunding site to help his bid to reach next year's Winter Olympics in PyeongChang, South Korea.
GB freestyle skiing head coach Pat Sharples: "Thinking of you Lloyd. Heal fast man. Positive vibes."
Olympic skeleton champion Amy Williams: "Praying for a speedy recovery to Lloyd Wallace."
Olympic skeleton silver medallist Shelley Rudman: "Sending best wishes for a speedy recovery Lloyd Wallace."
Ex-GB skier & Ski Sunday presenter Graham Bell: "Stay strong Lloyd Wallace, really hope you are okay."
British aerial skier Lloyd Wallace has suffered a "severe" head injury in a training crash in Switzerland.