We investigate the focus conditioning effects on molecular field-free alignment observed with high-order harmonic generation(HHG) from CO2 molecules.We also experimentally demonstrate that both the spectral shape and alignment signal of HHG significantly vary with changing focus position.A maximal alignment signal is achieved at a given focus position because of the optimal intensity of the driving laser.This intensity is related to the ionization potential of the molecules.These results indicate that a unique focus position provides an optimal alignment signal for practical applications. We investigate the focus conditioning effects on molecular field-free alignment observed with high-order harmonic generation (HHG) from CO2 molecules. We also experimentally demonstrate that both of the spectral shape and alignment signals of HHG vary vary with changing focus position. A maximal alignment signal is achieved at a given focus position because of the optimal intensity of the driving laser. This intensity is related to the ionization potential of the molecules. the result of that that unique focus position provides an optimal alignment signal for practical applications.