Effect patterns of food,predation and Apodemus agrarius as competition species on population dynamics of reed voles, Microtus fortis were measured using 2×2×2 factorial experiment design under the field enclosure. The independent effects of food availability, predation and interspecific competition on population MNKA were extremely significant. Except the marginally significant effect of interaction between predation and interspecific competition, the effects of other interaction between two or among three external factors on population density were all not significant; The independent effects of three external factors on population recruitment were all significant and the effects were additive; The different combination of food availability, predation and interspecific competition had extremely significant effects on proportions of reproductive females in the population; Three external factors had also extremely significant effect on proportions of juveniles and adults in the population. Comparing to the effects of predation and interspecific competition, the effects of food availability on age structure were weaker because they were all not significant in any combination of predation and interspecific competition; All of three external factors had significant effects on the growth rate of body mass in reed voles while their interactions had not; Results of MANOVA showed that the effects of predation on adult survival were most intense, those of food availability were less and those of interspecific competition were least, but the effects of interaction among three external factors were not significant. Except that the effects of preadation were marginally significant, the effects of food availability and interspecific competition on survival time of juvenile were not significant. the results of this dissertation provided sufficient evidence for the effects of food availability, predation and interspecific competition on population dynamics, and thus supported hypothesis that there were independent and additive effects of food availability, predation and intespecific competition on population dynamics.