Police said Friday the two men, whose names and nationalities were not disclosed, were performing a stunt involving an explosion on an inflatable boat on the Ognyanovo dam, just outside the capital, Sofia. The incident occurred late Thursday.
The injured man has undergone a five-hour operation and his condition was stable, doctors are quoted by the BTA news agency as saying.
In a statement, the production company Nu Image/Millennium Films confirmed the "unfortunate accident," adding "the filmmakers are working closely with the authorities."
The film stars Sylvester Stallone, Bruce Willis and Arnold Schwarzenegger.