Carboxymethyl groups were introduced into starch molecules to obtain a starch-based gastric floating drug carrier. The influence of degree of substitution (DS) of the carboxymethyl group on the structure and performance of the starch-based gastric floating drug carrier was investigated systematically. The results showed that, with increasing DS of the carboxymethyl group and decreasing crystallinity of carboxymethyl starch, the swelling property decreased after the initial increase; when the DS was 0.8, the swelling index of carboxymethyl starch increased by ten times compared with that of the native starch. Based on this result, a gastric floating drug delivery system was prepared with vitamin B1 as a functionally active substance and carboxymethyl starch as the carrier material. The in vitro release behavior of vitamin B1 gastric floating tablets was evaluated in simulated gastric fluid. The results revealed that the carboxymethyl starch based gastric floating tablets had good floating and sustained-release properties. When the carboxymethyl starch with a DS of 0.08 was used as the carrier, the tablets could maintain floating in gastric fluid for 8 h with a floating lag time of less than 2 min. The in vitro drug release kinetics suggest that the carboxymethyl starch based gastric floating system can extend the vitamin B1 release time to 6 h, and the release behavior follows zero order kinetics.