The invention discloses a determination device and method for an effective influence radius of a high-pressure medium injection drilling hole. The determination device includes a coal core holder, a high-pressure medium input and output portion, and a pressure test portion. The determination device and method only need to collect a core sample after a core drilling hole is constructed in an original coal seam, and perform determination in a laboratory without constructing a series of pressure test drilling holes in the coal seam, the engineering quantity is greatly reduced, and a testing success rate is higher than that of a traditional determination method; in addition, the determination method only needs a coal core with a small scale, pores and cracks in coal quality and the coal core are relatively uniform, an error of determination caused by unevenness of the coal quality and a geological structure can be avoided, and the coal core sample is sealed in the coal core holder, and themeasurement accuracy is high; and furtherly, through combination of a theoretical extrapolation method, it is not necessary to test a long coal pillar, test input and time can be greatly saved, and determination results can provide a theoretical basis and a data support for reasonable arrangement of the high-pressure medium injection drilling hole.