Cassava starch and polyvinly alcohol (PVA) were used to prepare a biodegradable foam composite by extrusion molding with other adjuvants. The table L16(45) was used to research the influence of starch content, mass fraction of NaHCO3 foamer, mixed plasticizer of glycerol and NaOH contents, temperature and shear rate on the rheological behavior of the composites melt. SEM was also used to observe the effect of different viscosity of melt on the cell morphologies. The results show that: the viscosity of melt significantly increased with the content of starch increasing, decreased with increasing of NaHCO3 content or glycerol and NaOH content. The viscosity of melt firstly decreased with the temperature and shear rate rising, but then increased. When temperature was 145 ℃ or shear rate was 500 s-1, the melt showed the lowest viscosity. The cell morphology of the starch/PVA composites showed the optimal value when the viscosity was 1 600 Pa·s.