For a type of automobile engine flywheel housing product,after processing two of its main round hole by the vertical lathe,as a result these two round holes turns out to be a roundness problem.Setting up a FEM model of the flywheel housing,the deformation data were acquired under the the centrifugal force as the workpiece rotate by using Altair Simlab and ANSYS Workbench to process a combined simulation.Analysis result indicates the centrifugal force produced by the workpiece is〖BF〗n’t〖BFQ〗 the main factor response for the roundness error,but for the runout of the spindle cause by the gap between inner ring and outer ring of spindle bearing,when workpiecefixture system centrifugal situation is existed.Based on the final result of the analysis,improvement scheme was made for the fixture.It provides reference for the reduction of the roundness error and improvement of product quality.