Abstract: A safety injection device has a barrel, a seat operatively and slidably received within the barrel and having a needle cannula extending out therefrom, a stopper detachably received in a chamber defined in the seat and a plunger slidably received in the barrel and having a pair of connectors detachably received in the stopper and a pair of spaced extensions The gap between the two extensions allows the plunger to engage with the stopper and therefore to push the seat originally received in the barrel. Due to the movement of the connectors, the seat is able to be secured with respect to the barrel with the needle cannula extending out from the barrel and pulled back into the barrel with the needle cannula received in the barrel. With such a construction, human intervention in the disposal and exposure to the contaminated cannula is avoided, and therefore, the risk of infection is reduced.