The invention relates to a secondary loop 3U0 line-breaking alarm apparatus. The apparatus comprises voltage transformers LHL connected to an L phase, an S phase, and an N3 phase of a main system and LHN connected to the L phase and the N3 phase, current relays LJL and LJN are arranged between secondary sides of the voltage transformers LHL and LHN and an L phase and an N3 phase of a relay protection chamber cable, and the L-S phase and the S-N3 phase are connected with load resistors 1FR and 2FR. When the condition is normal, the L-S phase voltage is 100 V, the S-N3 phase voltage is 100 V, and the current relays are in an action state; when phase failure of one phase of the L phase, the S phase, and the N3 phase occurs, return is performed, and alarm is given via a control loop. The apparatus is advantageous in that phase failure alarm is realized, misoperation is effectively prevented, and the maintenance is facilitated.