A safety disposable syringe includes: a barrel; a plunger extending into the barrel; a piston mounted on the plunger and formed with a first engaging member; and a needle seat extending fittingly into the barrel and having a front end disposed outwardly of the barrel, and a rear end disposed in the barrel and having a rear end face facing toward the piston. The needle seat defines a fluid passage extending through the rear end face of the rear end of the needle seat. The needle seat is formed with a second engaging member. The fluid passage has a terminating end terminated at the rear end face of the rear end of the needle seat. The second engaging member has a terminating end terminated at the rear end face of the rear end of the needle seat and spaced apart from the first terminating end of the fluid passage.