The head of the Pontifical Academy of Sciences (PAS) admitted he did not know whether the practice was continuing but said he hoped to encourage change.
Human rights groups say China is still using executed prisoners as a source of organ transplants.
Beijing says forced organ harvesting ended in 2015.
But the group Doctors Against Forced Organ Harvesting (DAFOH) said China's participation was compromising the conference and the PAS should reject its claim as no independent inspection had taken place.
