In order to understand the effects of long-term high-temperature on blood-related indexes and tissue structure of Nile tilapia （Oreochromis niloticus）， tail vein blood analysis and microscopic observation techniques were used to compare the hematological parameters and microstructural changes of the spleen and liver of Nile tilapia at normal-temperature （28 ℃） with those at long-term high-temperature domestication （34 ℃）. The results showed that the number of erythrocytes and hemoglobin content of tilapia in the high-temperature group were significantly higher than those in the normal-temperature group； although the morphology of hemocytes did not show any difference between the two groups of temperature conditions， the sizes of erythrocytes， monocytes， and neutrophils tended to decrease significantly in the high-temperature group. In addition， observations of tissue sections showed that long-term high-temperature domestication led to an increase in the number of erythrocytes， an expansion of the red pulp area， a relative decrease in the white pulp area， and an increase in the number of melano-macrophage in tilapia spleen tissues， as well as an intensification of hepatocyte vacuolization and an increase in the number of melano-macrophage in liver tissues. These results indicated that the long-term high-temperature environment had significant effects on blood indices and hematopoietic tissues of Nile tilapia， and it was hypothesized that they adapted to the high-temperature environment by regulating their hematopoietic function and immune response. This study provides an important reference for understanding the physiological adaptation mechanism of Nile tilapia under long-term high-temperature， and also provides a scientific basis for tilapia aquaculture practice in the context of global warming.