Abstract: In order to analyze the effects of preventing rib spalling by using water injection in extremely soft coal seam, the change of shear strength after water inj ection was carried out on extremely soft coal seam.Based on the theory of unsaturated soil theory and the direct shear tests, by using extremely soft coal samples whic h had been re-modelled, a series of direct shear tests with non consolidation and non drainage were performed under the conditions of different moisture contents and different normal stresses. The analysis method of shear strength was proposed in this paper, and the moisture content was used as the key index, the cohesion and int ernal friction angle were selected as the main indexes. The influences of moisture content on extremely soft coal seam were studied comprehensively combined with te sts and theoretical analysis.Results showed that in the condition of low normal stress, the shear strength of the sample tended to soften, and the sample was prone to h ardening in high stress state. Besides, the cohesion increased with the increasing of the moisture content, after that when the moisture content of the sample exceeded the optimum moisture content ( approximately 17.64%)，the cohesion began to decline and the moisture content possesses lttle effect on the internal friction angle. It i s proved that the engineering control technology of preventing rib spalling by using water injection in extremely soft coal seam is effective.