The group confirmed it has received complaints about the image of a blood-soaked zombie in an advertisement for Tulleys Farm Shocktober Fest, The Mirror reported Tuesday.
The advertisement can be seen on buses in London and Sussex County.
An ASA spokeswoman said officials are looking into whether a full investigation needs to be opened into whether the advertisement violates industry regulations.
Stuart Beare, organizer of the Tulleys Farm Shocktober Fest, said the image wasn't intended to be shocking.
"The image of the zombie girl on our advertising is no worse than many images you'll see at this time of year in newspapers and magazines, on the Halloween costumes in the supermarket aisles and even on kids' TV programs," he said.
"We have been running our Halloween events for over 15 years, and they are extremely popular, with up to 5,000 people a night going through the six haunted houses and rides.
"We are waiting for the Advertising Standards Authority to come back to us with their judgment to see if we have to take the ads off the buses."