The floods have displaced 20 million people, but donations are below those for catastrophes like the Haitian earthquake or the Asian tsunami.
Jolie said she understood that "it is getting hard for people - they see Haiti, they see these other events ... and they get exhausted by the time another big one rolls around."
But she said Pakistanis face "mass death, mass displacement, and this situation is going to get worse."
Jolie has visited Haiti, Iraq and other countries as a U.N. goodwill ambassador.
The star says she might visit Pakistan once "the cameras go away."
Jolie spoke Monday at the London premiere of her spy thriller "Salt."