Abstract: This paper describes the physical quantity — adjacent difference used to describe the relative change of geomagnetic field in Mobile seismic-geomagnetic monitoring in detail, We calculated the averages of 84 phases of the absolute adjacent difference of all measurement points in the monitoring area using mobile geomagnetic measurement data of western Yunnan region for 33 years. The average is called “Area Adjacent Difference”, and it is indicated by the symbol P_区, and the correspondence between P_区 and the earthquake is also analyzed. The results indicate that: It has obvious correspondence between P_区 and Earthquake occurred in and surrounding area——Most of the moderate and strong earthquakes occur in the low value period of P_区 in the measurement area、or after several periods of consecutive low values、or in the process of P_区 value declining. This conclusion provides a new analytical method for seismic-geomagnetic monitoring, and has certain reference significance for earthquake prediction.