A burglar alarm includes a main unit provided with a power supply voltage regulator circuit, a CPU circuit, an FET driving circuit, and an RLC oscillation circuit to convert digital code signals into analog signals, which are amplified and then carried via a power cord; and a decoding horn provided with a power supply voltage regulator circuit, a reference voltage circuit, an RLC oscillation circuit, a comparison circuit, a CPU circuit, and a horn voice driving circuit to decode and compare a received burglar-alarming signal and drive a horn to make vocal and sounding warnings. The burglar alarm is controlled via different frequencies that represent codes 1 and 0, or a base frequency and different pulse widths representing codes 1 and 0, so that the signals are carriable via the power cord without interference from noise on the power word.