The compressive behaviors of three-dimensional braided composites with different cross sections and lengths along braided direction under high strain rates were reported from numerical simulations and experimental tests. The microstructure models with and without defects were established. The microstructure model with random distributed defects was to investigate the influence of sample length and random defects on the compressive properties. The microstructure model without defects is to reveal the effect of the cross section area on the compressive behavior. We found from finite element analysis that volume fractions of interior, surface, and corner unit cells vary with the cross section area. The strength and modulus were sensitive to the volume fractions of the unit cells and defects in the braided composites. The effect of the sample length on the compressive behaviors was not as significant as the cross section area and defects. The testing validated the finite element analysis results well.