Abstract: The body weight of 2563 Apodemus agrarius have been analyzed in Yuqing, Guizhou Provence from 1987 to 2007. The results indicate that the minimum body weight is 5.36 gram, the maximum is 53.41 gram and the average is 26.31±8.14 gram. The body weight mostly ranges from 15.01 gram to 35.00 gram, which is 79.01% of the total, is stronger ability of the individual activities. The minimum body weight of pregnant Apodemus agrarius is 17.61 gram, and the minimum body weight of Apodemus agrarius whose testicle descend is 16.50 gram. The rats begin to propagate when their body weight reach 16.0 gram, with the body weight increase, the population reproductive capacity also increases. When the weight goes beyond 24.0 gram, the population reproductive capacity rapidly enhances. Meanwhile, the rate of pregnancy reach 31.82% and the descent rate of testicle reach 67.02%, which approach the total female rate of pregnancy 30.06% and the total male descent rate of testicle 52.67%. At the same time, the reproductive index reaches 0.7138, which approach the total reproductive index 0.7599. The population reproduction capacities of different age groups have remarkable difference. With the increment of population age, female rate of pregnancy, litter size, male descent rate of testicle, and reproductive index continuously increase. Adult group I, group II and the old group are the main reproductive contributors population of Apodemus agrarius. Therefore, the proportion of the three groups in the population is closely to the decrease or increase of the populations, which can be regarded as an important basis to predict the population quantities of the Apodemus agrarius