An adjustable supporting device includes a pivotal connection device and two supporting members individually pivotally connected to the pivotal connection device. The pivotal connection device includes a rotational restriction mechanism including a driving part and a restricting part connected to the driving part. The restricting part is capable of being driven by the driving part so as to move relative to the supporting members. Therein, the restricting part is capable of entering limitation ranges of rotational restriction structures of the supporting members simultaneously for individually limiting rotation ranges of the supporting members relative to the pivotal connection device according to the rotational restriction structures. Therefore, by designing the above rotation ranges, the adjustable supporting device can avoid damage to a user due to an unexpected folding of the supporting members, before the restricting part entirely departs from the limitation ranges of the rotation ranges of the supporting members.