A fan failure alarm device includes a first digital-analog converting circuit (200), a first diode (D11), a switching circuit (260), an input-output (I/O) controller (U11), and an alarm circuit (270). The first digital-analog converting circuit receives a digital pulse signal from a first fan, and outputs a direct current (DC) voltage signal at an output terminal. The first diode with a cathode coupled to the output terminal of the first digital-analog converting circuit, the first diode turns on when the first fan stops rotating. The switching circuit with an input terminal coupled to an anode of the first diode, outputs a first control signal when the fan rotates normally and outputs a second control signal when the fan stops rotating. The input-output (I/O) controller receives the first and second control signals from the switching circuit, and outputs an alarm signal when receives the second control signal. The alarm circuit receives the alarm signal from the I/O controller, and activates an alarm.