A synchronous folding device includes two opposing folding members and a multi-joint rotary axle structure mounted between the two folding members. The multi-joint rotary axle structure has two ends which can be folded or unfolded synchronously. The multi-joint rotary axle structure includes a driving joint assembly and a driven joint assembly. The driving joint assembly includes two opposing joint plates and a middle link plate. Two ends of the driving joint assembly are respectively connected to the two opposing folding members. Each joint plate of the driving joint assembly can be turned free through plural turning centers so that both ends can be closed or opened synchronously for the folding device to be folded or unfolded accurately.