The invention provides a sound spectrum based nondestructive testing device and method for small irregular castings. The device comprises a base, wherein an upright column is arranged on the base and sleeved with a fixing block, the fixing block is fixedly connected with a horizontal clamp, and a clamping part is arranged at the tail end of the clamp; a piezoelectric ceramic exciting source is arranged on one side of the clamping part, and an acoustic transducer is arranged on the base below the other side of the clamping part. During testing, a standard qualified casting and a to-be-tested casting are clamped in the clamping part of the clamp sequentially, the piezoelectric ceramic exciting source on one side of the clamping part is driven, vibration is produced under excitation and sound waves are emitted, the acoustic transducer on the base receives audio signals transmitted by air and transmits the signals to a host computer through a controller via an interface device, a standard resonance sound spectrum and a to-be-tested resonance sound spectrum are formed respectively, resonance frequency of the to-be-tested resonance sound spectrum is compared with resonance frequency of the standard resonance sound spectrum, and the to-be-tested casting is qualified if the resonance frequency is consistent; otherwise, the to-be-tested casting is unqualified.