A safety disposable syringe includes: a barrel (3)； a plunger (5) extending into the barrel (3)； a piston (6) mounted on the plunger (5) and formed with a first engaging member (611')； and a needle seat (4) extending fittingly into the barrel (3) and having a front end (41) disposed outwardly of the barrel (3), and a rear end (42) disposed in the barrel (3) and having a rear end face (420) facing toward the piston (6). The needle seat (4) defines a fluid passage (46) extending through the rear end face (420) of the rear end (42) of the needle seat (4). The needle seat (4) is formed with a second engaging member (44'). The fluid passage (46) has a terminating end (460) terminated at the rear end face (420) of the rear end (42) of the needle seat (4). The second engaging member (44') has a terminating end (440) terminated at the rear end face (420) of the rear end (42) of the needle seat (4) and spaced apart from the first terminating end (460) of the fluid passage (46).
