The invention discloses a one-time frequency modulation performance improvement test method considering the tolerance capability of a nuclear power unit. the method sequentially comprises a field test: a primary frequency modulation test and a transient test are carried out on the actual unit under different operating conditions, and a power adjustable range of the unit under the working condition is obtained; a simulation test is carried out on a full-process simulation machine of an actual unit, different dead zones, unequal rates and frequency modulation amplitude limiting amplitude are set under different power platforms, a loop is recorded, and the main parameters of the two-loop system are recorded, in the normal operation range, the dead zone and the frequency modulation amplitudelimiting amplitude are finely adjusted. According to the method, a dead zone and a frequency modulation amplitude limiting amplitude are reasonably set for different power platforms., so that the nuclear power unit can achieve primary frequency modulation response through heat storage of a conventional island steam turbine under the condition of small frequency difference; under the condition of large frequency difference, the performance of the primary frequency modulation response is achieved by adjusting the action amplitude and speed of the control rod.