An alarming method for a ventilator and a ventilator alarm system are provided. The method includes the following. First, a plurality of ventilation parameters are received and detected. If a peak airway pressure of the ventilation parameters conforms to a first condition, whether a plateau pressure conforms to a second condition is determined, wherein the first condition is Y(k+1), Ppeak>Yk, mean,Ppeak+3*Yk,sd,Ppeak, and the second condition is Y(k+1), Pplateau