Hydraulic flushing (HF), which uses water jet with high pressure to break coal, is widely used in the prevention and control of coal and gas (methane) outburst in coal mines. The key of HF technology is to determine the proper water pressure of water jet, otherwise, serious construction accidents will be induced during the period of water jet breaking coal. In this paper, coal, which is determined to be a typical piezoelectric material by the analysis the composition with X-ray diffraction method, can generate electromagnetic radiation (EMR) signal under external load because of the piezoelectric effect. Then, two mechanical stages of water jet breaking coal are analyzed theoretically to explore the relation between initial water pressure of water jet and EMR signal. Accordingly, a new method based on the analysis of EMR emitted from the deformation of coal under the action of water jet with different initial pressure is proposed to determine safe water pressure. The research results are as follows: (1) According to the field test, EMR has obvious regional characteristics. when the coal is impacted by water jet with safe pressure, maximum, average and frequency of EMR are more normal than that of invalid water pressure and dangerous water pressure;(2) From the perspective of coal rupture, the development of pores and fractures in the coal is fast and the macroscopic fracture scale, which is simulated in the same calculation steps, is large under the impact of water jet with safe pressure. Furthermore, the coal mass does not have a serious instability phenomenon, which means the suitability of safe water pressure ;(3) The fractal characteristics of EMR frequency which can present nonlinear and self-similarity characteristics of coal fracture process vary with pressure of water jet. When the initial water pressure of the water jet is safe, the value of the Delta alpha is smaller than that in invalid water pressure, but bigger than that in dangerous water pressure. This method has certain guiding significance for determining the safe water pressure of HF. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.