To improve the Signal-to-noise Ratio (SNR) and detecting probability of small target in infrared image sequences, a novel method of target detection based on spatial-temporal filtering is proposed. This method considered the real-time characteristic of the detecting algorithm. Firstly, the first frame was set as a reference frame and the other frames were compensated in motion. Secondly, the frames compensated were filtered by variance Weighted Information Entropy (Variance WIE) filter in spatial domain. After that, a bi-directional inter-frame difference method was adopted in temporal domain. At last, by detecting the protruding spot of the difference images, the background and noises in infrared images were inhibited and the small target was detected. The experimental results were analyzed and compared with some other methods. The results show that this approach can highly improve the SNR of target, precisely detect the small infrared target, and express fine real-time performance.