Objective:To observe the status of the sinus membrane using fiber optic endoscope during the lateral window approach sinus floor elevation to provide a reference for clinicians when evelvating the sinus mucoperiosteum.Methods:Sixty-six patients (72 sides) who underwent maxillary sinus floor elevation in Beijing Ruicheng Stomatology Hospital from September 2016 to December 2019 were selected, including 40 males and 26 females, aged 26-80 years old [(56.2±11.5) years]. And fiber optic endoscopy was used to observe the maxillary mucoperiosteum during the operation.Results:The status of maxillary sinus mucoperiosteal during lateral window approach sinus floor elevation can be divided into four categories: ① Class Ⅰ, complete periosteal, no damage to mucoperiosteum; ②Class Ⅱ, periosteal injury, unexposed laminae propria; ③Class Ⅲ, periosteal Rupture, exposed lamina propria; ④ Class Ⅳ, mucoperiosteum perforation, rupture of periosteum, lamina propria and epithelial layer. A total of 72 operations were performed, including 18 cases of class I, 28 cases of class Ⅱ, 4 cases of class Ⅲ, and 22 cases of class Ⅳ.Conclusions:The status of maxillary sinus mucoperiosteal during lateral window approach sinus floor elevation can be divided into four categories. Fiberoptic endoscopy as a clinical auxiliary examination method can improve the operator′s control of the status of the maxillary sinus membrane and assist the peeling of the mucosa.