Abstract: In order to discover the influence law of maximum stress of cassava tuber, and obtain the allowable maximumlifting force without the breakage of cassava tuber during cassava tuber lifting, the numerical simulation model of soil,cassava tuber and cassava stem system was established by adopting explicit dynamics simulation software LS DYNA.Inthe numerical simulation model, large deformations and fractures of the soils close to the tubers occurred and smalldeformations occurred in most parts of the rest.Thus, the coupling method of FEM(finite element method) and SPH(smoothed particle hydrodynamics) was applied in the numerical simulation model.The SPH method was used in inner soillayer where large deformations occurred and the FEM method was used in the outer soil layer where small deformationsoccurred.The coupling between the inner and outer soil was realized by“nodes surface” in LS DYNA.And MAT_FHWA_SOIL was selected as soil material model in the numerical simulation model, because it takes account of theinfluence of moisture content, strain softening, strain rate effect, void ratio, and pore water pressure, and obeys themodified Mohr Coulomb yield criterion.Numerical simulation tests of cassava tuber lifting were carried out by using thenumerical simulation model and the quadratic regression rotation design method.There were 5 experiment factors and 2experiment indices in the quadratic regression rotation design tests.The experiment factors were the lifting velocity, thelength, dimension and growth depth of the tuber and the soil hardness, respectively.The experiment indices were themaximum lifting force and the maximum stress of tuber which were measured under different factor combinationconditions.According to the numerical simulation tests results, the multi factor coupling mathematic models between themaximum stress of the tuber and the lifting velocity, the length, dimension and growth depth of the tuber and the soilhardness were established by using statistical software SPSS.Based on the coupling mathematic models, the differentinfluencing factors and their interactions on the maximum stress of the tuber were investigated, the relationship curveswere drawn by using mathematics software MathCAD, and the corresponding influence laws were obtained by therelationship curves.And based on the numerical simulation tests results, the scatter diagram of the maximum lifting forceand the maximum stress of tuber was plotted by using mathematics software MathCAD, and the correlation between themwas studied by the scatter diagram.According to the scatter diagram, the allowable maximum lifting force was obtainedunder which there was no breakage of tuber.Moreover, the allowable maximum lifting force under which there was nobreakage of tuber was verified by cassava tuber lifting tests, which were carried out in the cassava planting field in WumingCounty, Guangxi Zhuang Autonomous Region in the end of December 2014 by adopting the method of random sampling.Meanwhile, the maximum lifting force and the breakage rate of tuber were analyzed by statistical method.When theallowable maximum lifting force was less than 0.98 kN, the breakage rate of tuber was 2.5%.The result showed that the multi factor coupling mathematic models between the maximum stress of the tuber and the lifting velocity, the dimensionand growing depth of the tuber and the soil hardness had high precision, because the F test of the multi factor couplingmathematic models was significant at 0.000 1 level.The multi factor coupling mathematic models could be used in theeffect analysis of the maximum stress of tuber.With the increasing of lifting velocity, the maximum stress of tuberincreased at first and then decreased.With the increasing of growing depth, tuber length and soil hardness, the maximumstress of tuber increased.But the maximum stress of tuber decreased when tuber′s diameter increased.There was little correlation between the maximum stress of tuber and the allowable maximum lifting force.The allowable maximum liftingforce was 0.98 kN.