Sharptail sunfish, , is the most abundant Masturus lanceolatus species, and the catch is composed of 80-90% of Molidae in eastern Taiwan. The research center has studied the fishery and reproduction of sharptail sunfish since 2016. We studied the ontogenetic shifts and seasonal variation in feeding habits and trophic dynamics of sharptail sunfish using stable isotope analysis (SIA) and stomach content analysis. We collected the prey items of sharptail sunfish and constructed the food webs. We collected 121 stomach content samples and 213 muscle tissues of sharptail sunfish from 2016 to 2021. There were a wide variety of prey items in stomachs, including tunicates, crustaceans, and scyphozoan. Their diets were related to their body size and seasons. Large individuals can catch the prey with high mobility. Besides, results indicated that sharptail sunfish fed on benthic organisms in summer and autumn, showing that the coastal feeding might be related to their movement path. According to the results of SIA, sharptail sunfish had a dietary ontogenetic shift at 80 cm. The trophic level of individuals <80 cm was 3.8, and those of large individuals >80cm were 4.1-4.2. The study found that the feeding diets and trophic levels of sharptail sunfish shifted with seasonal resources, movement patterns and their energy requirements.