A dental handpiece comprising: a barrel defining an accommodating cavity therein and comprising an axial open end in communication with the accommodating cavity; an air turbine rotatably arranged in the accommodating cavity of the barrel; a central shaft secured to the air turbine to rotate with the air turbine; a first bearing mounted in the barrel adjacent the axial open end of the barrel and supporting the central shaft for rotation; and a seal being trumpet-shaped and comprising a sealing portion sleeved over an outer cylindrical surface of the central shaft and a mounting portion surrounding the sealing portion, the mounting portion being arranged and flattened between the open end of the barrel and the first bearing, wherein the sealing portion is deformable to separate from the outer cylindrical surface of the central shaft or closely contact the outer cylindrical surface of the central shaft depending on a pressure difference between interior and exterior of the barrel, wherein a first bearing seat is provided in the barrel, the first bearing seat is formed with a bearing hole and an annular groove at an axial end of the first bearing seat facing the air turbine, the first bearing is disposed in the bearing hole, the mounting portion is received in the annular groove and sandwiched between the first bearing and the first bearing seat, a depth of the annular groove in an axial direction of the barrel is smaller than a thickness of the mounting portion in the natural state of the mounting portion, wherein a flat gasket is provided between an axial end of the first bearing and the first bearing seat, and the mounting portion is sandwiched between the flat gasket and the first bearing seat, wherein the dental handpiece further comprises a second bearing and a second bearing seat, the first bearing and the second bearing are respectively arranged at opposite axial sides of the air turbine, the first bearing is disposed in the first bearing seat and close to the open end of the barrel while the second bearing is disposed in the second bearing seat and away from the open end of the barrel, and an elastic member is provided between an axial end of the second bearing and the second bearing seat, and wherein inner rings of the first bearing and second bearing clamp the air turbine, and the elastic member is compressed and deformed to generate a forward pre-tightening force, which acts on the seal via the second bearing, the air turbine, the first bearing and the flat gasket, such that the seal is clamped and flattened. a barrel defining an accommodating cavity therein and comprising an axial open end in communication with the accommodating cavity; an air turbine rotatably arranged in the accommodating cavity of the barrel; a central shaft secured to the air turbine to rotate with the air turbine; a first bearing mounted in the barrel adjacent the axial open end of the barrel and supporting the central shaft for rotation; and a seal being trumpet-shaped and comprising a sealing portion sleeved over an outer cylindrical surface of the central shaft and a mounting portion surrounding the sealing portion, the mounting portion being arranged and flattened between the open end of the barrel and the first bearing, wherein the sealing portion is deformable to separate from the outer cylindrical surface of the central shaft or closely contact the outer cylindrical surface of the central shaft depending on a pressure difference between interior and exterior of the barrel, wherein a first bearing seat is provided in the barrel, the first bearing seat is formed with a bearing hole and an annular groove at an axial end of the first bearing seat facing the air turbine, the first bearing is disposed in the bearing hole, the mounting portion is received in the annular groove and sandwiched between the first bearing and the first bearing seat, a depth of the annular groove in an axial direction of the barrel is smaller than a thickness of the mounting portion in the natural state of the mounting portion, wherein a flat gasket is provided between an axial end of the first bearing and the first bearing seat, and the mounting portion is sandwiched between the flat gasket and the first bearing seat, wherein the dental handpiece further comprises a second bearing and a second bearing seat, the first bearing and the second bearing are respectively arranged at opposite axial sides of the air turbine, the first bearing is disposed in the first bearing seat and close to the open end of the barrel while the second bearing is disposed in the second bearing seat and away from the open end of the barrel, and an elastic member is provided between an axial end of the second bearing and the second bearing seat, and wherein inner rings of the first bearing and second bearing clamp the air turbine, and the elastic member is compressed and deformed to generate a forward pre-tightening force, which acts on the seal via the second bearing, the air turbine, the first bearing and the flat gasket, such that the seal is clamped and flattened.