The US actress was due to visit Assange on Tuesday, before holding a press conference outside Belmarsh Prison in south-east London.
However a statement from the Don’t Extradite Assange campaign said: “Pamela has a personal emergency which has meant she has had to go home and cancel everything for the next two weeks.”
Anderson says WikiLeaks founder Assange has been subjected to “traumatic psychological torture” as she urged his home country Australia to stop him being extradited to the US.
The former Baywatch star, one of Assange’s most high-profile supporters, visited him in jail earlier this year.
She recently said she was “extremely distressed at the thought of his declining state” and added that her heart broke for him.
Anderson tweeted that Assange was “hanging on like a super hero” in Belmarsh, where he was sent earlier this year after spending seven years in the Ecuadorean embassy in London to avoid extradition.
Assange was jailed for 50 weeks in May for breaching his bail conditions after going into hiding to avoid extradition to Sweden over sex offence allegations.
He faces 18 charges in the US, including allegations that he conspired to break into a Pentagon computer and worked with former US army intelligence analyst Chelsea Manning to leak hundreds of thousands of classified documents.