The embodiment of the invention provides a main transformer protective device test method and a portable arithmetic facility. In the embodiment of the invention, all arithmetic equations corresponding to the protective devices with different models of different manufacturers are pre-stored in the portable arithmetic facility; test staff only need to set the manufacturers, the models of the protective devices and the parameter values of the protective devices; and the tested currents are respectively compared with the brake current, the current magnitude needed by each side of a main transformer is automatically computed by the portable arithmetic facility and the errors can be automatically computed. According to the technical scheme disclosed by the embodiment of the invention, the test can be finished under the condition that the test staff do not need to correctly understand the protective principle and formula, and the test precision is improved due to no effect of human memory, various computations are automatically finished by the portable arithmetic facility, and the test time is reduced in comparison with manual computation.