The following issues have been debated： ① The principle on zero risk of flood control safety design for water structures may lead to a new concept of limiting flood. ② There are four special properties in limiting flood. ③ A new idea of probable limiting flood has been given. ④ The relation between limiting flood and 10000-year flood characterized of the unity of opposites. ⑤ To determine a limiting value of flood in practice, the rational analysis is always important.