An infrared small target detection algorithm based on wavelet transform and improved Top-Hat filter is proposed.Firstly,the images were decomposed by the single level wavelet transform to obtain four sub-band wavelet coefficients represented as approximate,horizontal,vertical and diagonal respectively.And then the approximate sub-band was filtered by the improved Top-Hat method,and the difference image was obtained through the difference of filtering results and original approximate sub-band.The filtered approximate sub-band was fused with the horizontal sub-band to obtain new approximate sub-band and horizontal sub-band.The coefficients of vertical sub-band and diagonal sub-band were set to zeros simultaneously.Afterwards,the wavelet transform was reconstructed.To make the targets more prominent,the reconstructed image was enhanced by the grey transformation based on histogram.The experimental results indicate that this method can accurately detect the infrared small target and has good robustness.