Abstract: In order to study the hole stability of horizontal wells in a shale formation, it is necessary to accurately and completely evaluate the rock strength in shale reservoirs. Based on the characteristics of shale reservoirs in the Longmaxi Formation, multiple evaluation methods (e.g. compression test, buckle hardness test and scratch test) were used to evaluate and analyze the compressive strength of shale. It is shown that the compressive strength of Longmaxi shale is generally in the range of 100-300 MPa, with an average of 123.71 MPa. After the shale has a contact with water, its compressive strength around the micro cracks fall by about 80%. Research showed that the compressive strength of Longmaxi shale was more influenced by the relative enrichment of mineral composition,the formations with high content of quartz and low content of clay mineral have higher compressive strength.The effect of micro cracks on strength of shale is a dominant characteristic. When water based drilling fluids are used for drilling horizontal wells in shale formation, it is necessary to strengthen the sealing capacity in micro cracks so as to effectively lower the risk of hole instability.