The research on the formation damage mechanism of conventional and unconventional oil and gas reservoirs is relatively well-established， but there has been a lack of attention towards the type classification and formation damage mechanism of geothermal reservoirs. Currently， only two types of geothermal reservoirs are recognized in the exploration and development of geothermal resources， namely， sandstone and karst. Because of the unclear concept and type of reservoir， the incorrect drilling method， drilling fluid selection and wrong resource assessment are caused. Furthermore， there is no formation damage mechanism and damage assessment criteria， so that the geothermal well completion process is missing， and leads to small water volume， low temperature， and even abandonment. On the basis of a large number of geothermal engineering， this paper focuses on the research of geothermal reservoir types and formation damage mechanism， aiming at the outstanding problems and referring to the conventional petroleum reservoirs research results. Researches show that geothermal reservoirs can be categorized into four types， which are porous， fissure， karst， and compound type. This categorization allows for more accurate drilling methods， selection of drilling fluids， and resource assessment. The main modes of thermal reservoir damage are porosity blockage or failure in the structure； The damage caused by intrusion from drilling fluid as well as static and dynamic pressure from liquid column within wells are the primary factors and inevitable common problems. The formation damage types of geothermal reservoirs can be divided into five categories： stress damage， flow rate damage， water sensitivity damage， alkali sensitivity damage and acid sensitivity damage.