Abstract: Based on the Improved Quantum Molecular Dynamics model (ImQMD), the difference of central collision in experiment and theory for low-intermediate energy heavy ion collisions is investigated. By analyzing the two kinds of central collisions, we find there is 50% difference on the charge distribution, and also on its average kinetic energy for heavier fragments, especially at the beam energy of 50 MeV/u. While, for the coalescence invariant neutron proton yield ratio, the calculations show there are about 5% ~ 15% difference. Our results evidence that the impact parameter smearing effects must be considered for tightly constraining the nuclear equation of state and the in-medium nucleon-nucleon cross sections by heavy ion collision.