Refraction index mismatch between flat left-handed metamaterial(LHM)lens and its surrounding medium generally destroys the focusing of flat LHM lens and degrades the performance of near-field target detection by usingflat LHM lens.For LHM lens of refraction index mismatch within±30%,numerical simulations demonstrate that lenses with large refraction index may suffer less resolution degradation than lenses with small refraction index,and the enhancement of refocused microwave backscattered from target can be subsided bly up to approximately 5.5 dB.The refraction index mismatch will also shift the target position in the reconstructed image so that theoretical prediction of target position needs to be modified.