"The deal is: You're going to have to p--- off everybody in the United States or you're not going to get it done," the former senator from Wyoming told "Squawk on the Street."
Trump has said his plans will pay for themselves by spurring economic growth that drives up tax revenue. Simpson doesn't buy that.
"You're not going to get economic growth of 4 percent," he said. "That's Disney World."
Simpson and Democrat Erskine Bowles were co-chairmen of the National Commission on Fiscal Responsibility and Reform. In 2010, the bipartisan Simpson-Bowles commission recommended slashing entitlements and increasing taxes, but its proposals to cut the national debt fell short of adoption.
Simpson said his plan was on the right track — fixing the debt will require entitlement cuts because programs such as Social Security and Medicare are "going broke." Groups that don't push for reform are doing a disservice to future generations by passing down a broken system, he added.
"Where is the AARP?" he asked during the phone interview, arguing that the retiree group should be encouraging Social Security reform. "They don't give a whit about their grandchildren."
He said Trump's proposed cuts targeting groups that receive relatively small amounts of federal funding are symbolic and will have little impact on the debt crisis. The national debt is nearly $20 trillion.
"Everybody knows what we have to do, and the rest of this stuff is fantasy. You know, cutting NPR, and the humanities and the arts. Give me a break. That's almost disgustingly funny. It just makes no sense."