Abstract: To study the precision of the constrained and unconstraint mode dimension reduction model for flexible manipulator, the finite element method is applied to describe the flexible deformation, the Lagrange method is used to establish the dynamical model; and then the dimension is reduced by the constrained and unconstrained mode method; in the end, the absolute errors of the two dimensional reduction method are analyzed. The simulation results show that both models have a good performance in the precision of dynamics. The unconstraint mode dynamic model has a higher precision when considering the same order, the absolute error of the constraint mode model has the nature of accumulation over time.