Hot dry rock (HDR) geothermal energy is a green and renewable resource, and its efficient development has always been a hot research topic worldwide. Using the natural fracture system of deep HDR as a reservoir not only reduces the difficulty and developmental cost of artificial reservoir construction but also greatly increases the efficiency of water rock heat exchange. In a study of the characteristics of the natural fracture system of deep HDR, through field observations, it was found that structural fractures in deep granite bodies filled by magma or hydrothermal fluid are common phenomena. Through observations with a polarising microscope, it was found that under the high-temperature and hydrothermal fluid action of the backfill, changes in the number of thermal-cracking fractures of fracture-filled granite versus the distance from the cementation interface can be divided into three zones: a zone in which the number of fractures in the backfill fluctuates gently, a zone showing a sharp increase in the number of fractures in the parent rock, and a zone in which the fluctuation in the number of fractures in the parent rock decreases. The position of the maximum number of thermal-cracking fractures is located in the parent rock at a certain distance away from the cementation interface. This maximum number has an exponential relation with the thickness of the backfill, and its position is also related to the thickness of the backfill. The universality of the fracture backfill and its large-range influence on the granite parent rock caused by thermal action result in the formation of a huge weak-plane structure in the granite parent rock. This weak-plane structure is very likely to become a natural reservoir in HDR geothermal development or an easy-to-rupture position when an artificial reservoir is constructed by hydraulic fracturing, which is beneficial for the construction of artificial reservoirs. Based on this result, we studied a new technology for the construction of artificial reservoirs for HDR geothermal exploitation, which constitutes a new research direction for HDR geothermal development. The shorter version: The study of the characteristics of the natural fracture system of deep hot dry rock (HDR) constitutes a new research direction for HDR geothermal development. Through field observations, it was found that structural fractures in deep granite bodies filled by magma or hydrothermal fluid are common phenomena. Through micro-scale observations, it was found that under the high temperature and hydrothermal fluid action of the backfill, the changes in the number of thermal cracking fractures of fracture-filled granite versus the distance from the cementation interface can be divided into three zones. The position of the maximum number of thermal-cracking fractures is located in the parent rock at a certain distance away from the cementation interface. This maximum number and its position are both related to the thickness of the backfill. The huge weak-plane structure caused by thermal action is very likely to become a natural reservoir or an easy-to-rupture position when an artificial reservoir is constructed by hydraulic fracturing, which is beneficial for the construction of artificial reservoirs. Based on this, we studied a new technology for the construction of artificial reservoirs for HDR geothermal exploitation. (C) 2019 Elsevier Ltd. All rights reserved.