The invention relates to the technical field of monitoring of plunger pumps, in particular to a method for preventing evacuation of a plunger pump based on monitoring of the output pressure. The method relates to the plunger and an output pipeline, and is characterized in that a pressure data acquisition control device is arranged on the outlet pipeline; and a data processing method comprises the following steps that (1) the average value P0 of the output pressure is obtained, specifically, the output pressure setting tolerance delta P is set, the domain above a P0+delta P line is called a peak domain, and the domain below a P0-delta P line is called a valley domain; a peak domain counter A and a valley domain counter B are set correspondingly; the allowable difference value between the peak domain counter A and the valley domain counter B is the numerical value C; and an out-of-tolerance time counter D and an upper limit E are set; (2) pressure data Px is acquired; (3) abnormal data are removed; (4) the number of times of Px falling into the peak domain and the valley domain is calculated; and (5) whether the out-of-tolerance time D of the difference value between the peak domain counter A and the valley domain counter B exceeds a preset value E or not is judged, and if yes, an evacuation alarm is given out. Compared with a detection method in the prior art, the method is accurate and reliable, and false alarms are avoided.