Firefighters were called to Wimborne Road, Winton, in Bournemouth, after the display caught fire on Christmas Day.
Neighbours evacuated homes as Dorset Fire and Rescue put the fire out.
A spokesman said: "At the risk of sounding like a Christmas tale, the cause of the fire was... sun refracting through a snow globe which then ignited the reindeer food."
He added: "There was severe fire damage to the shop window display, heat damage to the window and severe smoke damage to the entire shop."
A fire investigator described the incident as "rare", but said it was possible because of the sun's low position in the sky.
"Consideration should be given to the possibility that this could happen in your homes, particularly with shaving-type mirrors which can focus the suns rays and cause fires," the spokesman added.
The happened at about 11:30 GMT, with fire engines from Redhill Park and Springbourne tackling the fire.
Fake reindeer food went up in flames in a charity shop window after sunlight was refracted through a snowglobe.