The invention discloses an occasional fault diagnosis method of a field bus. The occasional fault diagnosis method comprises steps as follows: a master station is provided with an effective communication cycle counter used for recording the cycle number of effective cycle data frames transmitted by the master station in the normal communication stage of the field bus as well as two reception counters used for recording the cycle number of the correct and effective cycle data frames received from a port A and a port B of the master station; each slave station is provided with two reception counters used for recording the cycle number of the correct and effective cycle data frames received from a port A and a port B of the slave station; the master station judges whether each slave station receives the correct and effective cycle data frames in the communication cycle according to CRC (cyclic redundancy check) of the received effective cycle data frames; when an occasional communication fault occurs to the field bus, the master station enters a fault processing state; the master station positions communication connection with the occasional communication fault according to numerical differences of a bus topology structure, the reception counters of the master station and the reception counters of the slave stations. With the adoption of the method, connection with the occasional fault in the field bus of a numerical control system can be quickly and accurately positioned.