Scanning electron microscopy along with energy dispersive spectroscopy and sample-electrolysing method was employed to investigate the causes for the formation of cellular salient defects on the surface of galvanized steel sheets. Results show that the skin defect of cold rolled steel sheet is the direct cause of the surface cellular salient defects of galvanized sheets. The strip of silicate inclusions larger than 300 μm located at the subsurface area of the cold rolled steel sheet，and massive iron oxide lied among silicate inclusions. Mechanical grinding was conducted to remove the subsurface area of the pickling plate with the same defect batch，and the subsequent cold rolling test showed that the main reason of the surface defects of the galvanized sheet was not caused by massive iron oxide inclusions. The inclusion content in the casting billet of galvanized steel sheets with defects was 100.32 mg/10 kg which was composed of inclusions larger than 140 μm，while the normal value was 20.98 mg/10 kg for the casting billet of galvanized steel sheets without defects. The results proved that the fundamental cause of the surface cellular salient defects of galvanized steel sheets was the large silicate inclusions.