The invention disclose a line breaking type antitheft alarm which comprises a transformer, a rectifying circuit, a switch, a magnetically controlled switch, a magnet, a first light-emitting diode, a second light-emitting diode, a relay, a time base integrated chip, a four-tone warning chip, a voltage stabilizing tube, a loudspeaker, a first diode, a second diode, a third diode, a fourth diode, a fifth diode, a first resistor, a second resistor, a third resistor, a fourth resistor, a fifth resistor, a sixth resistor, a seventh resistor, an eighth resistor, a first capacitor, a second capacitor, a third capacitor, a triode and a battery. When the magnetically controlled switch is turned off, a power supply rapidly charges the second capacitor through the second resistor and a sixth diode, and voltage of the trigger end of and voltage of the reset locking end of a time base integrated circuit also rise; when the voltage of the trigger end of and the voltage of the reset locking end are higher than a threshold value, the output end outputs a low level, a warning circuit works, and the loudspeaker gives an alarm. The line breaking type warning is adopted, a line breaking type warning mode has a breakage-proof function, and the alarm can also be given even if a signal line is destroyed; the line breaking type antitheft alarm is simple in circuit structure, adopted elements are fewer, so that the cost is low, and the line breaking type antitheft alarm is favorable for popularization.