A lifeguard alarm system for a swimming pool has a sensing device, at least one electromagnetic wave receiver and a host, the sensing device capable of attaching onto a user; wherein when the sensing device detects the more than two conditions, an microprocessor in the sensing device sends a signal to the at least one electromagnetic wave receiver disposed around the swimming pool and wirelessly connected to the host, and the host sends out an alarm. Furthermore, includes a fail-safe mechanism; therefore, when the first detecting mechanism of the sensing device detects that the user is not in the corresponding predetermined dangerous state, to prevent malfunctioning of the first detecting mechanism from creating problems.