A syringe with a blocking structure, comprising: a barrel, having a through hole formed at a front end of the barrel, and a barrel inverted hook plate extended in a direction slightly downwardly towards the through hole and warped to the outside; a circular body, with a square cross-section and a specific compressibility, and disposed in the barrel at the bottom of an end of the through hole; a needle seat, disposed at an end of the through hole of the barrel, and having a head holder section and a fitting section, the periphery of the head holder section having a first circular portion and a second circular portion, a circular groove being formed between the first circular portion and the second circular portion, and the front edge of the first circular portion abutting the circular body, wherein when the barrel inverted hook plate is warped and extended into the circular groove it is blocked by the rear edge of the first circular portion, the fitting section extending from the head holder section to the exterior of the through hole; and a rod, comprising a rod body section, an engagement section extending from the rod body section to be accommodated in the needle seat, characterised in that: the barrel has a plurality of dovetail grooves uniformly formed at an end of the through hole and in the barrel, each dovetail groove passing from the barrel axially into an end of the through hole, a barrel inverted hook plate being disposed deeply at each dovetail groove in the barrel and at a position further than the internal side of the dovetail groove, and each barrel inverted hook plate having a dovetail member disposed at an end and fitting in the corresponding dovetail groove, wherein the periphery of the second circular portion can push away the barrel inverted hook plate and drive the dovetail member into the dovetail groove, the head holder section at the rear of the second circular portion abuts a seal ring of the barrel, and the rod comprises a hollow cork cap which covers the engagement section and can be plugged into the needle seat. a barrel, having a through hole formed at a front end of the barrel, and a barrel inverted hook plate extended in a direction slightly downwardly towards the through hole and warped to the outside; a circular body, with a square cross-section and a specific compressibility, and disposed in the barrel at the bottom of an end of the through hole; a needle seat, disposed at an end of the through hole of the barrel, and having a head holder section and a fitting section, the periphery of the head holder section having a first circular portion and a second circular portion, a circular groove being formed between the first circular portion and the second circular portion, and the front edge of the first circular portion abutting the circular body, wherein when the barrel inverted hook plate is warped and extended into the circular groove it is blocked by the rear edge of the first circular portion, the fitting section extending from the head holder section to the exterior of the through hole; and a rod, comprising a rod body section, an engagement section extending from the rod body section to be accommodated in the needle seat, characterised in that: the barrel has a plurality of dovetail grooves uniformly formed at an end of the through hole and in the barrel, each dovetail groove passing from the barrel axially into an end of the through hole, a barrel inverted hook plate being disposed deeply at each dovetail groove in the barrel and at a position further than the internal side of the dovetail groove, and each barrel inverted hook plate having a dovetail member disposed at an end and fitting in the corresponding dovetail groove, wherein the periphery of the second circular portion can push away the barrel inverted hook plate and drive the dovetail member into the dovetail groove, the head holder section at the rear of the second circular portion abuts a seal ring of the barrel, and the rod comprises a hollow cork cap which covers the engagement section and can be plugged into the needle seat. the barrel has a plurality of dovetail grooves uniformly formed at an end of the through hole and in the barrel, each dovetail groove passing from the barrel axially into an end of the through hole, a barrel inverted hook plate being disposed deeply at each dovetail groove in the barrel and at a position further than the internal side of the dovetail groove, and each barrel inverted hook plate having a dovetail member disposed at an end and fitting in the corresponding dovetail groove, wherein the periphery of the second circular portion can push away the barrel inverted hook plate and drive the dovetail member into the dovetail groove, the head holder section at the rear of the second circular portion abuts a seal ring of the barrel, and the rod comprises a hollow cork cap which covers the engagement section and can be plugged into the needle seat.