A trocar, comprising: a trocar sleeve comprising a bowl-shaped proximal end housing part, a distal end portion, and a sheath portion between the two; a sealing structure comprising a top shell having an outer wall and a bottom shell integral with each other, wherein the said sealing structure can be connected to the proximal end housing portion of the trocar sleeve in a sealed manner and is removable, thus enabling the bottom shell of the sealing structure to be contained in the proximal end housing portion of the trocar sleeve and the outer wall of the top shell of the sealing structure to be exposed proximally to the proximal end housing portion of the trocar sleeve when the trocar is assembled; and an obturator comprising a central shaft and a proximal end portion which is located at the proximal end of the central shaft and adapted to be held by a surgeon, wherein the axial direction of the trocar is defined by the extension direction of the central shaft which also has a obturator tip at the distal end to perform puncturing, wherein the central shaft runs through the sheath portion of the trocar sleeve and the sealing structure in a sealed manner, the obturator tip of the central shaft extends out of the distal end portion of the trocar sleeve and the proximal end portion of the obturator is positioned proximally to the top shell of the sealing structure when the trocar is assembled; wherein the proximal end portion of the obturator, the top shell of the sealing structure and the proximal end housing portion of the trocar sleeve form a head of the trocar when the trocar is assembled, the axial dimensions of the proximal end portion of the obturator, the top shell of the sealing structure and the proximal end housing portion of the trocar sleeve in the axial direction of the trocar respectively are d1, d2 and d3, and d3 is equal to or greater than 1/2 of the sum of d1, d2 and d3; and characterized in that the top shell of the sealing structure is provided with a valve for controlling the opening and closing of a gas passage for the trocar; the valve comprising a valve body, a valve core within the valve body, and an operating handle capable of driving the valve core to rotate in the valve body, wherein the valve body is integral with the top shell of the sealing structure, the valve core extends in an approximately transverse direction perpendicular to the axial direction of the trocar or approximately in a direction forming an acute angle with the ends of the sealing structure, and the extension direction of the operating handle is approximately perpendicular to that of the valve core. a trocar sleeve comprising a bowl-shaped proximal end housing part, a distal end portion, and a sheath portion between the two; a sealing structure comprising a top shell having an outer wall and a bottom shell integral with each other, wherein the said sealing structure can be connected to the proximal end housing portion of the trocar sleeve in a sealed manner and is removable, thus enabling the bottom shell of the sealing structure to be contained in the proximal end housing portion of the trocar sleeve and the outer wall of the top shell of the sealing structure to be exposed proximally to the proximal end housing portion of the trocar sleeve when the trocar is assembled; and an obturator comprising a central shaft and a proximal end portion which is located at the proximal end of the central shaft and adapted to be held by a surgeon, wherein the axial direction of the trocar is defined by the extension direction of the central shaft which also has a obturator tip at the distal end to perform puncturing, wherein the central shaft runs through the sheath portion of the trocar sleeve and the sealing structure in a sealed manner, the obturator tip of the central shaft extends out of the distal end portion of the trocar sleeve and the proximal end portion of the obturator is positioned proximally to the top shell of the sealing structure when the trocar is assembled; wherein the proximal end portion of the obturator, the top shell of the sealing structure and the proximal end housing portion of the trocar sleeve form a head of the trocar when the trocar is assembled, the axial dimensions of the proximal end portion of the obturator, the top shell of the sealing structure and the proximal end housing portion of the trocar sleeve in the axial direction of the trocar respectively are d1, d2 and d3, and d3 is equal to or greater than 1/2 of the sum of d1, d2 and d3; and characterized in that the top shell of the sealing structure is provided with a valve for controlling the opening and closing of a gas passage for the trocar; the valve comprising a valve body, a valve core within the valve body, and an operating handle capable of driving the valve core to rotate in the valve body, wherein the valve body is integral with the top shell of the sealing structure, the valve core extends in an approximately transverse direction perpendicular to the axial direction of the trocar or approximately in a direction forming an acute angle with the ends of the sealing structure, and the extension direction of the operating handle is approximately perpendicular to that of the valve core.