To have a quick temper
Definition 1 of 1
Pro Tip 1/3Emotional Reactivity
Describes someone's tendency to become angry swiftly without much provocation. 
She avoided confrontational topics with Paul, knowing his quick temper.
Pro Tip 2/3Potential Offense
Use cautiously as 'quick temper' could be offensive if used to describe someone directly. 
I didn't say he had a quick temper; I just said he's passionate about his views.
Pro Tip 3/3Not Just Anger
Implies the person may also act impulsively when angry, sometimes regretting it later. 
Her quick temper made her quit the job on the spot, but she apologized the next day.