George Farrell was serving a four year jail term for burglary offences against his former partner and being held at HMP Northumberland.
Newcastle Crown Court heard the 28-year-old confessed to prison officials during a meeting that he wanted to kill the woman, who is from Sunderland.
Prosecutor Stuart Graham told the court: "He talked openly about blaming his ex partner for him being in prison.
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"He stated he wanted to kill her by eating parts of her.
"Further, he claimed he wished to take her severed head to the police station, so that he could go to prison for something he had done.
"He also said he would kill his mother and daughter so they wouldn't have to know the person he had become."
Farrell, of no fixed address but is originally from the Sunderland area, denied making threats to kill and claimed things he said had been "misconstrued" but was found guilty by a jury after a trial.
Judge Amanda Rippon said: "I am quite satisfied when he said it, he meant it.
"If he had been free to hurt her she would have been in serious difficulty, serious trouble."
Judge Rippon said Farrell should be classified as a dangerous offender and sentenced him to five years behind bars with a four year extended licence period.
The judge told him: "You told people in the room you were going to kill her by eating parts of her and would cut her head off and take it to the police station so you could be sentenced for something you had actually done."
Judge Rippon said the threats were taken "very seriously indeed" and the victim, who was informed of what happened, suffered significant distress and psychological harm.
The court heard Farrell has mental health problems including an emotionally unstable personality disorder.
Judge Rippon said she accepted the threats were not made directly to the victim.
The court heard since 2017 Farrell has committed offences of possessing bladed article, assault, breach of restraining order and burglaries in relation to the same victim.