A foldable car seat carriage includes a frame unit and an engagement unit. The frame unit includes two leg frames. Each of the leg frames includes two legs pivotable relative to each other, and a driving mechanism. The driving mechanism has opposite ends connected pivotally to the legs at first and second intersections. The engagement unit includes two bayonets. Each of the bayonets has a linking portion connected pivotally to the driving mechanism of a respective one of the leg frames. For each of the leg frames, change of a distance between the first and second intersections of the driving mechanism and the legs during pivot action of the legs converts a position of the engagement unit relative to the legs.