A pump failure alarm device includes two digital-analog converting circuits (100, 110), a voltage sampling circuit (120), a controlling circuit (130), an I/O controller (U11), and an alarm circuit (140). The digital-analog converting circuits receive digital pulse signals from two pumps (200, 210), and respectively output a DC voltage signal at an output terminal when the pumps run normally. The voltage sampling circuit with two input terminals respectively coupled to the output terminals of the two digital-analog converting circuits, and outputs a first voltage signal when the two pumps run normally and outputs a second voltage signal when either or both of the pumps stop running. The controlling circuit receives the voltage signals from the voltage sampling circuit, and outputs a first control signal when it receives the first voltage signal and outputs a second control signal when it receives the second voltage signal. The I/O controller receives the control signals from the controlling circuit, and outputs an alarm signal when it receives the second control signal. The alarm circuit receives the alarm signal from the I/O controller, and activates an alarm.