It is commonly observed in solar flares that the conjugate footpoints of newlyreconnected magnetic fields have different hard X-ray (HXR) emissions.Such HXR footpointasymmetry can be affected by several effects simultaneously,such as the acceleration process inthe corona or the plasma and magnetic field conditions during the transportation process.In thisstudy,we attempt to address the properties of photospheric magnetic fields in the areas coincidedwith asymmetric HXR footpoints based on RHESSI flare observations during 2002-2009.Ourstatistical results show that the strong HXR footpoint is preferentially (75％) appear at the weakmagnetic field region,which is qualitatively consistent with the asymmetric magnetic mirrorscenario.In addition,the 2004 November 4 M5.4 flare and the 2004 November 6 M3.6 flare arestudied in details to illustrate the different possibilities causing the asymmetry reversal.Bycomparing the estimated asymmetry quantities with HXR lightcurves,the asymmetry reversal inthe late period of M5.4 flare is mainly attributed to the difference of coronal energy release oracceleration processes in different periods,while it is associated with the locations of HXRfootpoints moving to the different magnetic field regions in the M3.6 flare.