A method for detecting open phase of a startup/standby transformer based on optical current transformer, CT, comprising the following steps: step 1: using optical CT to measure a three-phase current on a high-voltage side of the startup/standby transformer; step 2: receiving, by an open phase detection device, a current signal output by the optical CT, calculating the three-phase current on the high-voltage side of the startup/standby transformer in real time, and determining whether an open phase fault occurs according to the current; and step 3: if it is determined that an open phase fault occurs, issuing an alarm signal after a set delay or acting to trip, wherein the specific method for determining whether an open phase fault occurs in step 2 comprises when the current of a certain phase is lower than a set threshold value, determining that the open phase fault occurs in the phase; and the identification formula is as follows: I p < k 1 × I p.0 in formula, I p is the real-time calculated fundamental amplitude of the current of a certain phase on the high-voltage side of the startup/standby transformer, I p.0 is the fundamental amplitude of the current of the phase on the high-voltage side of the startup/standby transformer under a no-load condition, charaterised in that k1 is a reliability coefficient ranging from 0.5 to 0.8 and step 2 further comprises: when the current of a certain phase on the high-voltage side of the startup/standby transformer does not satisfy the symmetry, determining that the open phase fault occurs in the phase; and the identification formulas are as follows: I A • < k 2 × I B • − I C • I B • < k 2 × I C • − I A • I C • < k 2 × I A • − I B • formulas,, and respectively correspond to phase-A, -B, and -C disconnections, wherein İ A, İ B, and İ C are the real-time calculated fundamental phasors of the three-phase current on the high-voltage side of the startup/standby transformer, and k2 is a reliability coefficient ranging from 0.25 to 0.4. step 1: using optical CT to measure a three-phase current on a high-voltage side of the startup/standby transformer; step 2: receiving, by an open phase detection device, a current signal output by the optical CT, calculating the three-phase current on the high-voltage side of the startup/standby transformer in real time, and determining whether an open phase fault occurs according to the current; and step 3: if it is determined that an open phase fault occurs, issuing an alarm signal after a set delay or acting to trip, wherein the specific method for determining whether an open phase fault occurs in step 2 comprises when the current of a certain phase is lower than a set threshold value, determining that the open phase fault occurs in the phase; and the identification formula is as follows: I p < k 1 × I p.0 in formula, I p is the real-time calculated fundamental amplitude of the current of a certain phase on the high-voltage side of the startup/standby transformer, I p.0 is the fundamental amplitude of the current of the phase on the high-voltage side of the startup/standby transformer under a no-load condition, charaterised in that k1 is a reliability coefficient ranging from 0.5 to 0.8 and step 2 further comprises: when the current of a certain phase on the high-voltage side of the startup/standby transformer does not satisfy the symmetry, determining that the open phase fault occurs in the phase; and the identification formulas are as follows: I A • < k 2 × I B • − I C • I B • < k 2 × I C • − I A • I C • < k 2 × I A • − I B • formulas,, and respectively correspond to phase-A, -B, and -C disconnections, wherein İ A, İ B, and İ C are the real-time calculated fundamental phasors of the three-phase current on the high-voltage side of the startup/standby transformer, and k2 is a reliability coefficient ranging from 0.25 to 0.4.