A connecting device includes a case (1), a first rod (2), a second rod (3), and a third rod (4) that are connected to the case (1). Two first connecting members (5) connect the second rod (3) and the third rod (4) to the case (1). A second connecting member (6) connects the first rod (2) to the case (1). The case (1) includes an upper housing (101) and a lower housing (102) that define a first receiving hole (10), a second receiving hole (11), and a third receiving hole (9), the second connecting member (6) and the two first connecting members (5) are received in the first receiving hole (10), the second receiving hole (11), and the third receiving hole (9), respectively.