To solve the problem of the disturbance of the background edge under complex cloud background,an infrared small target detection method based on SVM posterior probability is proposed.In this method,infrared small target detection is considered as a binary classification problem between the target and the background.According to the characteristic of infrared image,the gradients in 8 directions of every pixel are taken as the classifying basis of the target and background.And the gradients that can respectively reflect the characteristics of target and background are chosen to be the main part of the training set,so that the classification model can be acquired after training.The gradients in 8 directions of every pixel are taken as the test set to obtain the posterior probability of SVM,which is exactly the output of detection,thus the targets are extracted from the image.The experimental results show that the algorithm is effective.