A new algorithm for detecting dim moving target in IR image sequence by the infinite norm of discontinuous frame difference vector is p roposed in this paper. Based on discontinuous frame difference, the detection system can detect not only the dim moving targetwhose disp lacement is no less than one p ixel between two continuous frames, but also the dim moving one whose disp lacement is no more than one p ixel between two continuous frames and whose gathered disp lacement ofmultip le continuous frames is larger than one pixel. These provide a new solution to detectmultip le targets with different velocities in IR image sequence. Simulations of real IR image sequence with low SNR p rove the algorithm can effectively detect dim moving target, and has high detection probability and excellent real time performance.