The invention relates to a method for determining the failure of an alternating-current power detecting unit. The method comprises: step one, a voltage value or a current value outputted by an alternating-current power detecting unit is sampled regularly at fixed time interval Ts; after comparison of each sample value with a reference value, the sampled value is transformed into a binary bit code 1 or 0, and the binary bit code is stored into a storage unit by bits, wherein the A sampled values are sampled totally within a determination cycle and the determination cycle A*Ts is not less than a cycle T of the alternating-current power; step two, bit-based reading and statistics are carried out on the numbers of binary bit codes 1 and 0 in the storage unit and the number are marked as B and C; and step three, the B and the C are compared with a threshold or |B-C| is compared with the threshold; and when at least one of the B and C is smaller than the threshold or the |B-C| is larger than the threshold, the failure of the alternating-current power detecting unit is determined, wherein the |B-C| is an absolute value of the difference between the B and C. With the method provided by the invention, determination on the failure of an alternating-current power detecting unit is realized.