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With the capacity to reduce wave energy and trap sediment, Scirpus mariqueter has become an important native species of annual grass for ecology restoration at the Yangtze Estuary in eastern China. Due to seasonal variances of biophysical characteristics, S. mariqueter usually bends and breaks in winter, resulting in flattened stems that may reduce its wave attenuation capacity. To investigate the effects of vegetation flattening on wave attenuation, a set of flume experiments were conducted for flattened and standing vegetation under different wave conditions. The model vegetation was designed to represent the wilted S. mariqueter collected in winter with dynamic similarity. Results showed that the wave damping coefficient for flattened vegetation (βF) was 33.6%-72.4% of that for standing vegetation (βS) with the same vegetation length. Both βF and βS increased with wave height but decreased with water depth. A wave attenuation indicator (WAI) was defined to generate empirical formulas for βS and βF as well as their ratio βF/βS. The empirical formulas were then applied to modify the existing standing vegetation-based wave attenuation model for flattened vegetation and performed successfully. Understanding the wave attenuation characteristics of flattened vegetation is essential for the management of ecological restoration and coastal protection.