An exemplary height-adjusting device (10) includes a base (12), a sliding block (14) and at least one fixing member (18). The base has a sliding slot (122) defined therein and a first sidewall (123), a second sidewall (124), a third sidewall (125), and a fourth sidewall (126) surrounding the sliding slot. At least one fixing hole (1232) defined in the first sidewall. The at least one fixing hole communicates with the sliding slot. The sliding block is slidably received in the sliding slot and partly protruding outwardly from the sliding slot. At least one fixing member cooperates with the at least one fixing hole to selectively fix the sliding block in the base.