Scanning electron microscopy was used to observe the morphological structure of small intestinal mucosa in embryos of Quail (Coturnix coturnix), Red-billed Gull (Larus ridibundus) and Black-winged Stilt (Himantopus himantopus). The results showed that during embryonic development of the quail and the red-billed gull, the morphological structure of the small intestine mucosa experienced three stages. The first stage was from the 10th to 11th day of embryonic development in the quail and from the 13th to 14th day of embryonic development in the red-billed gull (equivalent to 60% of embryonic development), when the small intestine mucosa was ridge-like longitudinal plica (Fig. 1a, d, Fig. 2c); The second stage was from the 12th to 13th day of embryonic development in the quail and from the 15th to 16th day of embryonic development in the red-billed gull (equivalent to 70% of embryonic development), when the small intestine mucosa transformed into "W"-shaped fold (Fig. 1b, 1e, Fig. 2a); The third stage was from the 14th to 17th day of embryonic development in the quail and from the 17th to 22th day of embryonic development in the red-billed gull (equivalent to 80% of embryonic development), when the small intestinal mucosa showed finger-like structure (Fig. 1c, f, Fig. 2b, d). On the 10th day of incubation (equivalent to 60% of embryonic development), the small intestinal mucosa of the black-winged stilt was ridge-like longitudinal plica (Fig. 1g, Fig. 2e); From the 12th day of incubation (equivalent to 70% of embryonic development) to hatching, the small intestinal mucosa of the black-winged stilt maintained "W"-shaped fold (Fig. 1h, Fig. 2f). We preliminarily judge that the regular change of embryonic intestinal mucosal structure of birds belongs to the iterate of ancestral phylogeny.