People as third-party observers, without direct self-interest, may punish norm violators to maintain social norms. However, third-party judgment and the follow-up punishment might be susceptible to the way we frame (i.e., verbally describe) a norm violation. We conducted a behavioral and a neuroimaging experiment to investigate the above phenomenon, which we call the "third-party framing effect". In these experiments, participants observed an anonymous perpetrator deciding whether to keep her/his economic benefit while exposing a victim to a risk of physical pain (described as "harming others" in one condition and "not helping others" in the other condition), then they had a chance to punish that perpetrator at their own cost. Our results showed that the participants were more willing to execute third-party punishment under the harm frame compared to the help frame, manifesting a framing effect. Self-reported anger toward perpetrators mediated the relationship between empathy toward victims and the framing effect. Meanwhile, activation of the insula mediated the relationship between mid-cingulate cortex activation and the framing effect; the functional connectivity between these regions significantly predicted the size of the framing effect. These findings shed light on the psychological and neural mechanisms of the third-party framing effect.