【Objective】To solve the problem of lodging with density increase in the soybean production improving process and the soybean lodging-resistance characters in different population structures. 【Method】Five levels of planting densities were designed to study the lodging-resistance mechanisms of stem and root selecting the new soybean varieties of Jinong30 and Jinong40 as the tested materials. 【Result and conclusion】Density could change the inner structure of plant population. With the population density increasing， lodging aggravated gradually and the difference was significant compared with the low density. In the high density population structure， the internode length became obviously long compared with the low density； the fresh mass unit stem length of basal internodes was lower than that of upper internodes， and low density was opposite. With the density increasing， the decreasing range of basal stem diameter and fresh mass was more obvious than that of upper internodes. The matter distribution of stem basal from the first to the fifth internodes was changed， which weakened the lodging resistance of the plant. The stem lodging-resistance characters were as follows： with the plant density increasing， the stem crushing strength， stem lodging-resistant index and fresh mass unit stem length decreased significantly； there were significantly negative correlations between all above indexes and lodging； the plant height/stem diameter， height of gravity center， internode length/diameter increased significantly， and there were significant positive correlations. The root nodule number， root dry mass and bleeding amount of soybeans decreased with the density increasing， and there were negative correlations， the correlations between lodging series and bleeding amount and root dry mass reached an extremely significant level with the highest correlation coefficients. The experiment showed that the above-ground stem crushing strength， tem lodging-resistant index， the root bleeding amount and root dry mass most closely correlated with lodging， which was feasible to measure the plant lodging-resistance capacity. Moreover， the soybean plant had a certain self-recovering capability under mild and moderate lodging conditions.