Passive millimeter wave (PMMW) imagers are increasingly becoming practical sensor candidates for target detection tasks. This paper is devoted to the ship detection by land-based W band passive polarized imager. The radiation characteristics of sky, sea surface and the atmospheric absorption characteristics of sea area near Qingdao are calculated based on sounding data. And the W band radiation characteristic of painted metal is measured. Then, the experiments of detection for sea surface ship are carried out in different ranges including 2,5, and 15 kilometers. The results show that land-based PMMW imager can detect and locate ship target from sea surface more than tens of kilometers away in thick fog, and polarization selection has influence on detection performance.