A support base for a child safety seat includes a shell body, an adjustable arm assembled with the shell body and having a rear end provided with a fastener operable to attach with an anchorage fixture of a vehicle, a safety lock, and a coupling portion assembled through an interior of the adjustable arm. The shell body is movable along the adjustable arm to adjust a distance between the fastener and a rear of the shell body. The safety lock is assembled with the shell body, and includes a latch operable to engage and disengage an opening of the adjustable arm. The latch is operable to lock the shell body with the adjustable arm, and a locking engagement of the fastener with the anchorage fixture causes a displacement of the coupling portion that pushes the latch to disengage from the opening for allowing displacement of the shell body toward the fastener.