The present invention provides a loss-of-field protection method for a phase modifier based on a degree of influence on a system. The method includes: a first reactive power Q output by the phase modifier and a commutation bus voltage U of an electrical power system are collected; a fundamental component of the first reactive power Q is filtered out to obtain a numerical range of a second reactivepower according to the commutation bus voltage U; fault conditions of the electrical power system are divided into a first fault, a second fault and a third fault; and the protection action corresponding to the fault condition is adopted according to the range of the second reactive power under each fault condition. According to the technical solution of the embodiment of the present invention, the influence of field loss of the phase modifier on different operating conditions of the system can be judged by using only the reactive voltage of the phase modifier and the bus voltage. Compared with the prior art, the implementation cost is low; the feasibility is strong; and action can be correctly and sensitively taken to the phase modifier with loss-of-field.