Industry tracker Exhibitor Relations made the estimate, even though the film has not yet opened in China.
If confirmed, the superhero sequel would overtake The Fate of the Furious's $542m global opening weekend haul, a record set in 2017.
The film is also seen taking the all-time domestic opening record, making $250m in the US alone.
Official figures will come out on Monday, but this would put Avengers ahead of Star Wars: The Force Awakens' $248m take.
Infinity War, directed by brothers Joe and Anthony Russo, cost between $300m and $400m to make.
