Sean Marshall swung the weapon across his partner's face during the attack at her home in Wakefield.
Marshall, 34, later told police he had no memory of inflicting the wound because he was so drunk.
Leeds Crown Court heard Marshall attacked his partner of eight years in the early hours of December 14 last year.
Carmel Pearson, prosecuting, said the woman had been out drinking with friends and contacted Marshall when she returned.
Miss Pearson said it was not clear what led to Marshall carrying out the attack.
The prosecutor said: "The defendant came towards her with a knife and slashed her.
"She remembers him swinging the knife across the bottom of her face and blood going everywhere."
The woman ran to a nearby petrol station to get help.
She was taken to Pinderfields Hospital to have the three-inch wide cut treated by surgeons.
The court heard the woman may be left permanently scarred.
Marshall, of Newstead Avenue, Outwood, pleaded guilty to unlawful wounding.
He has previous convictions for assault and affray.
"His account about not remembering is true.
"When he was found outside the house nearby he was in such a state that he was biting the top off a bottle and was bleeding to the mouth.
"He was quite clearly out of it on this occasion."
Mr Morley said Marshall was disgusted with himself for carrying out the attack.
Jailing Marshall, Judge Robin Mairs said: "The knife had sliced her chin.
"I have seen the pictures of the horrific wounds which will inevitably lead to scarring."
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