Abstract: With the help of sedimentology, paleontology, and stratigraphy, 14 sequence boundaries were recognized and 15 third-order sequences divided in the Xihu Sag, and each sequence is a response to the sea level change. Sequences from the Eocene to Pliocene were divided with the boundaries of unconformity or conformity equivalent to unconformity, which matches well the key boundaries in the curve of sea level change and is supported by palaeontological records.