A safety syringe assembly and a replaceable base unit are disclosed. The replaceable base unit of the present invention includes a base portion, a needle cover and a disengagement actuator. The base portion is configured to detachably connect with a syringe holder and has a through-hole structure, which is used to connect with an external needle. The needle covers proximal end is connected with the base portion to allow the needle cover to move with respect to the base portion, enabling it to contain and cover the syringe needle. The disengagement actuator is configured on the base portion and disengages the base portion from the syringe holder in pressing mode.
