The small target detection in infrared imagery with complex background of sea and sky is studied. Taking into account that small target of infrared image guidance like naval vessel is always nearby the sea-sky-line, wavelet multiresolution edge detection algorithm is adopted to detect the sea-sky-line. The potential target area can be decided subsequently, then the target is found by careful search in this area. Therefore, a new algorithm for small target detection from coarse scale to elaborate scale is proposed. Experimental results indicate that the method can detect and locate small targets precisely in infrared image obtained in conventional noise conditions. The correct detection ratio of single image is more than 96%, and this method has high detection ratio from a single image and high speed.