The invention discloses a shield grid oxide layer of a shield grid-deep groove MOSFET and a formation method thereof. The method comprises the following steps: 1, grooves are formed in an epitaxial layer through etching; first oxide layers form on the bottom surfaces and the sidewall surfaces of the grooves through adoption thermooxidizing growth technology; 2, nitridation layers form on surfaces of the first oxide layers; 3, deposited polysilicon layers form on the surfaces of the nitridation layers through adoption of polysilicon deposition technology; 4, thermooxidizing is carried out, and the deposited polysilicon layers are all oxidized to form second oxides; and the first oxide layers, the nitridation layers and the second oxide layers together form a shield grid oxide layer. According to the invention, an existing mode of directly depositing oxide layers on nitridation layers through SACVD is replaced by a mode of depositing polysilicon on the nitridation layers and carrying out thermooxidizing on the polysilicon, so that the thickness of each shield grid oxide layer used for isolation between shield grids and the groove sidewalls and between the shield grids and the groove bottom portions is ensured; technology conditions are simplified; the production cost is reduced; and the production power is raised.