The utility model provides a pressure shortage alarm for a fire extinguisher. The pressure shortage alarm comprises a magnetic pointer, a bidirectional thyristor, a button switch, a power supply and an early warning device. The pressure shortage alarm for the fire extinguisher is arranged on a pressure gauge of the fire extinguisher, the magnetic pointer coincides with a pointer of the pressure gauge of the fire extinguisher, the pointer of the pressure gauge of the fire extinguisher can rotate to drive the magnetic pointer to rotate, the power supply is connected with the center of the magnetic pointer and a T2 electrode of the bidirectional thyristor, a negative electrode of the power supply is connected with a negative electrode of the early warning device, a positive electrode of the early warning device is connected with one end of the button switch, the other end of the button switch is connected with a T1 electrode of the bidirectional thyristor, and a G electrode of the bidirectional thyristor is led out to be in contact with the magnetic pointer in rotation. The pressure shortage alarm for the fire extinguisher is arranged on the pressure gauge of the fire extinguisher. If pressure in the fire extinguisher is abnormal, early warning can be sent out right now. Thus, the fire extinguisher can be replaced by people timely, and the situation that the fire extinguisher cannot be used when fire happens is avoided.