Abstract: Tungsten is an important plasma-oriented material in fusion reactor. The study of radiation damage effect of pure tungsten under H/He mixed beam irradiation is of great significance for understanding the service behavior of this material in fusion reactor. In this work, high purity tungsten samples were irradiated with H/He mixed beam at 500 ºC, and three different H/He irradiation fluences. After the implantation experiment, nano-indentation instrument and field emission transmission electron microscope (TEM) were used to measure the changes of hardness with depth and the microscopic defects of the samples before and after irradiation. The results show that the ion irradiation will lead to the hardening of tungsten under all irradiation fluences. With the increase of irradiation fluence, the hardness of the sample increases first and then decreases. TEM results show that irradiation damage results in the formation of bubbles and dislocation loops. The average diameter of the bubble increases from 1.03 to 1.12 nm and then decreases to 1.05 nm. The number density of the bubble increases with the increase of the irradiation fluence. The dislocation size increases from 4.12 to 5.39 nm, but the dislocation loop number density is reduced by almost half. Finally, the relationship between hardness change and microstructure evolution is discussed. It is found that the hardness change after H/He mixed beam injection is mainly related to dislocation loops.