A pump body assembly comprising: at least two structure members, a cylinder arranged between the two structure members, and a piston assembly arranged in the cylinder ; wherein the piston assembly comprises a piston sleeve and a piston slidably arranged in the piston sleeve ; a position-limiting protrusion is provided on a lower end surface of the piston sleeve ; the position-limiting protrusion fits another structure member located below the cylinder to prevent the piston sleeve from moving in the radial direction relative to the other structure member, the at least two structure members comprise a lower flange and a lower position-limiting plate ; the lower position-limiting plate and the lower flange are both disposed below the cylinder ; the lower position-limiting plate is disposed between the cylinder and the lower flange ; the position-limiting protrusion is limited and stopped by the lower position-limiting plate to prevent the piston sleeve from moving in the radial direction relative to the lower position-limiting plate ; characterized in that a second extended part is provided on the surface of the lower flange , and the surface of the lower flange faces the piston sleeve ; the second extended part limits and stops the position-limiting protrusion to prevent the piston sleeve from moving in the radial direction relative to the lower flange ; the second extended part is located inside the position-limiting protrusion ; the position-limiting protrusion extends into a central hole of the lower position-limiting plate , fits and is limited by an inner surface of the central hole of the lower position-limiting plate ; and an upper end surface of the piston sleeve fits and is limited by a lower end surface of one structure member disposed above the piston sleeve to prevent the piston sleeve from moving in a radial direction relative to the one structure member. at least two structure members, a cylinder arranged between the two structure members, and a piston assembly arranged in the cylinder ; wherein the piston assembly comprises a piston sleeve and a piston slidably arranged in the piston sleeve ; a position-limiting protrusion is provided on a lower end surface of the piston sleeve ; the position-limiting protrusion fits another structure member located below the cylinder to prevent the piston sleeve from moving in the radial direction relative to the other structure member, the at least two structure members comprise a lower flange and a lower position-limiting plate ; the lower position-limiting plate and the lower flange are both disposed below the cylinder ; the lower position-limiting plate is disposed between the cylinder and the lower flange ; the position-limiting protrusion is limited and stopped by the lower position-limiting plate to prevent the piston sleeve from moving in the radial direction relative to the lower position-limiting plate ; characterized in that a second extended part is provided on the surface of the lower flange , and the surface of the lower flange faces the piston sleeve ; the second extended part limits and stops the position-limiting protrusion to prevent the piston sleeve from moving in the radial direction relative to the lower flange ; the second extended part is located inside the position-limiting protrusion ; the position-limiting protrusion extends into a central hole of the lower position-limiting plate , fits and is limited by an inner surface of the central hole of the lower position-limiting plate ; and an upper end surface of the piston sleeve fits and is limited by a lower end surface of one structure member disposed above the piston sleeve to prevent the piston sleeve from moving in a radial direction relative to the one structure member. <EOS>