Craig Christy, 48, and his son, Shawn Christy, 20, are expected to appear in federal court in Anchorage on Monday to change their pleas from not guilty, prosecutor Retta-Rae Randall said Friday.
Sentencing has not been set.
By signing the plea agreement, Craig Christy, of McAdoo, Penn., avoided what could have been a two-year prison sentence and a $250,000 fine.
The documents state that he agreed to be placed on five years of probation and have no contact with certain people whose names were not disclosed.
The Christys were arrested in August. Prosecutors say they were upset about restraining orders issued by an Alaska magistrate on behalf of Palin, the former Alaska governor and 2008 Republican presidential nominee, along with her family and friends.
A restraining order was issued against Shawn Christy because he was accused of stalking Palin.
The father's restraining order was issued following accusations that he left antagonizing telephone messages with Palin's parents.
The Christys later acknowledged making harassing phone calls from Pennsylvania to Palin's lawyer.
In one obscenity-laden communication, Craig Christy threatened to kill lawyer John Tiemessen, the criminal complaint says. It says Tiemessen's offices in Anchorage and Fairbanks received hundreds of calls, sometimes in one day, with some of the calls involving threats against Tiemessen and Palin.
"Defendant knew that his actions would clog the telephone lines of the law firm and waste their time," the plea agreement states.
A call to Christy's lawyer, James Wendt, was not immediately returned Friday.
Earlier this week, Shawn Christy, who had strongly contested the protective order issued against him for allegedly stalking Palin, agreed to plead guilty to making harassing phone calls to Palin's lawyers under a similar plea agreement.
In one message he threatened to come to Alaska and rape one of the attorneys, court documents say.
Shawn Christy's plea agreement also calls for five years of probation, no jail time and conditions similar to those imposed on his father.
Under the plea deals, the Christys also cannot possess or use a computer or cellphone with access to online services without prior approval of his probation officer. If law enforcement or probation officers suspect the Christys have violated the conditions, they have agreed to allow searches of themselves, home and property at any time, the plea agreement says.
Both also will be required to complete a mental health treatment program.