The Republican presidential nominee has signaled he will soften his immigration plan, which was a central plank of his primary campaign.
Instead of sending all 11 million people living illegally in the US, he now says only criminals will go.
Mrs Palin’s backing of Trump in January was regarded as a coup.
She demonstrated as John McCain’s running mate in 2008 that she possesses a rare star power in the Republican party.
But on Friday she told the Wall Street Journal that “wishy-washy positions” on core positions would result in “massive disappointment”.
“Parts of the message we heard in the last week are clearly not consistent with the stringent position and message that supporters have received all along,” she said.
The New York businessman made his tough line on immigration central to his win in the primary contests, a triumph that was unexpected when he launched his campaign with a controversial attack on Mexican immigrants as “rapists”.
He often derided Republican rivals Jeb Bush and Marco Rubio as weak on immigration and his plan to build a wall on the Mexican border became a chant at his rallies.
But this week he has openly talked about how tough it is to break up families and said people who have been in the US a long time and not broken any laws should stay.
He has not backed down on the wall, but staunch conservatives like Ann Coulter and Rush Limbaugh have also expressed concerns about his change of stance on deportations.
Democratic candidate Hillary Clinton dismissed his new policy as a “a desperate effort” while Mr Bush called Trump’s repositioning “abhorrent”.
“I can only say that whatever his views are this morning, they might change this afternoon, and they were different than they were last night, and they’ll be different tomorrow,” said the former Florida governor.
But his new campaign manager, Kellyanne Conway, insists “nothing has changed in terms of the policies”.
Mr Trump is expected to outline his new immigration policy in a speech next week, after postponing one that was due this week in Colorado.