A diagnostic device includes a data obtaining module and an analyzing module. The data obtaining module is configured to obtain a NC program block and receive condition data of an external device corresponds to the NC program block at the same time when the external device performs a NC program. The NC program block is a NC code of the NC program, the external device comprises a plurality of peripheral equipments, and the NC program block corresponds to at least one peripheral equipment of the peripheral equipments. If the condition data is abnormal, the analyzing module is configured to determine the at least one peripheral equipment of the peripheral equipments is abnormal based on the NC program block corresponding to the condition data which is abnormal.