A method for manufacturing a trench split-gate device, comprising: etching a semiconductor substrate to form a vertical upper half trench; etching the semiconductor substrate obliquely downward from a bottom of the upper half trench to form a lower half trench extending downward from the bottom of the upper half trench and with a width that increases from top to bottom, and a bottom of the lower half trench being concave arc-shaped, and a trench being constituted by the upper half trench and the lower half trench together; forming a first oxide layer on an inner surface of the trench; etching the first oxide layer to enable a side wall of the first oxide layer vertical up and down; forming a second oxide layer on the first oxide layer at the bottom of the lower half trench using high density plasma chemical vapor deposition process, so that a floating gate oxide layer formed by combination of the first oxide layer and the second oxide layer thickens from top to bottom along a side wall of the lower half trench, and the thickness of the floating gate oxide layer at a lower part of the side wall of the lower half trench is the same as that of the floating gate oxide layer at the bottom of the lower half trench (222; depositing polysilicon into the trench to form a floating gate polycrystalline layer; growing an insulating medium on an upper surface of the floating gate polycrystalline layer to form an isolation layer; and forming a control gate on the isolation layer in the trench. etching a semiconductor substrate to form a vertical upper half trench; etching the semiconductor substrate obliquely downward from a bottom of the upper half trench to form a lower half trench extending downward from the bottom of the upper half trench and with a width that increases from top to bottom, and a bottom of the lower half trench being concave arc-shaped, and a trench being constituted by the upper half trench and the lower half trench together; forming a first oxide layer on an inner surface of the trench; etching the first oxide layer to enable a side wall of the first oxide layer vertical up and down; forming a second oxide layer on the first oxide layer at the bottom of the lower half trench using high density plasma chemical vapor deposition process, so that a floating gate oxide layer formed by combination of the first oxide layer and the second oxide layer thickens from top to bottom along a side wall of the lower half trench, and the thickness of the floating gate oxide layer at a lower part of the side wall of the lower half trench is the same as that of the floating gate oxide layer at the bottom of the lower half trench (222; depositing polysilicon into the trench to form a floating gate polycrystalline layer; growing an insulating medium on an upper surface of the floating gate polycrystalline layer to form an isolation layer; and forming a control gate on the isolation layer in the trench. etching the first oxide layer to enable a side wall of the first oxide layer vertical up and down; forming a second oxide layer on the first oxide layer at the bottom of the lower half trench using high density plasma chemical vapor deposition process, so that a floating gate oxide layer formed by combination of the first oxide layer and the second oxide layer thickens from top to bottom along a side wall of the lower half trench, and the thickness of the floating gate oxide layer at a lower part of the side wall of the lower half trench is the same as that of the floating gate oxide layer at the bottom of the lower half trench (222; depositing polysilicon into the trench to form a floating gate polycrystalline layer; growing an insulating medium on an upper surface of the floating gate polycrystalline layer to form an isolation layer; and forming a control gate on the isolation layer in the trench.