Injecting kinetic hydrate inhibitors (KHIs) is an efficient option to prevent gas hydrate blockages in equipment and pipelines, which widely exist in petroleum production and transportation operations. Polyvinyl caprolactam(PVCap) is a kind of effective and commercialized KHIs. In this paper, one kind of carboxyl-acid-group-modified PVCap named carboxyl terminated polyvinyl caprolactam (PVCSCOOH) has been synthesized and the KHI performance has also been evaluated. The microscopic properties for hydrate samples were detected with cryo-scanning electron microscopy, Raman spectroscopy, and powder X-ray diffraction. It was found that the maximum subcooling degree of 2 wt % PVCSCOOH not generating methane hydrate was 13.7 degrees C, while this value for PVCap was only 11.3 degrees C; PVCSCOOH had a better inhibition performance than PVCap of similar molecular weight. In addition, we also noticed that PVCSCOOH could change the hydrate appearance and lower the large/small cavity ratio (L/S), to 2.00 but it had no impact on the hydrate structure.