A new background suppression method based on combined shearlet transform and Bayesian mechanism was proposed to solve the problem which is dim and small target detection contained complex sky clouds and ground background clutter for infrared search and tracking system. Firstly, according to difference of distributed charateristiscs between target and background clutter,in infrared image. the shearlet transform was adopted to decompose the origimal infrared image into multi-scale and multi-direction, which extracts multi-scale and multi-direction detail features of origimal image. Then, Gaussian scale mixture (GSM) model was introduced to separate dim, small target and background clutter from infrared image for suppression background. Finally, target image was obtained by using classical adaptive thresholding segmentation technique and target detection implemented. When compared with two dimensional least mean square (TDLMS) method, several groups of experimental results demonstrate that the proposed method can suppress complicated background in dim small target image effectively.