A child safety seat alarm system includes a child safety seat, an alarming mechanism, a detection mechanism, a trigger mechanism and a driving portion. The detection mechanism is configured to detect whether both states of the alarming mechanism and the detection mechanism meet a predetermined criterion. The trigger mechanism is switchable between an open state and a closed state. The detection mechanism is in a non-working state when the trigger mechanism is in the open state. The detection mechanism is in a working state when the trigger mechanism is in the closed state. When the child is seated in the child safety seat, the driving portion is operated to switch the trigger mechanism from the open state to the closed state so as to switch the detection mechanism to the working state. When the detection mechanism detects the states of the alarming mechanism and the detection mechanism meet the predetermined criterion, the alarming mechanism outputs an alarm signal for alarming. The child safety seat alarm system can remind a user whether or not a child is left in a car, this would prevent a dangerous scenario that the child is left in the car.