A clamping device (100) for clamping a portable electronic device is provided. The clamping device includes a mounting board (12) and a clamping mechanism (14). The clamping mechanism includes at least one adjusting member (141), at least one sliding block (145), and a sliding rail (146). The sliding rail is selectively attached to the mounting board, either horizontally or vertically. Each respective sliding block is slidably mounted to the sliding rail. Each adjusting member respectively engages with a corresponding sliding block and is configured to drive the corresponding sliding block to move along the sliding rail.