The invention relates to a transformer protection CT disconnection discrimination method, a differential protection method and a device. The discrimination method comprises the steps that if the onside of the transformer satisfies a three-seconds wiring mode, three-phase phase current and three-phase spill current of the onside of the transformer and a middle circuit breaker, and zero-sequence current corresponding to the side and middle circuit breakers is calculated; following conditions are judged: 1) a minimum value of the three-phase phase current of the onside circuit breaker or the middle circuit breaker is less than a first set threshold, a maximum value of the three phase phase current is greater than a second set threshold and less than a third set threshold; 2) the zero-sequencecurrent of the side circuit breaker or middle circuit breaker and the sum of the zero-sequence current of the side and middle circuit breakers are all greater than a fourth set threshold; 3) the maximum value of the three-phase differential current is greater than a fifth set threshold; if the above conditions are all satisfied, it is determined that the onside of a CT is disconnected. The methodcan quickly and accurately determine whether the CT of the transformer side is disconnected, and provide a reliable basis for accurate movement of transformer differential protection.