Stress arch effect has influence on the calculation of stress and effective stress in reservoirs，and the effective stress has influence on the analysis result of stress sensitivity of porosity and permeability. The stress arch ratio is usually used to characterize the degree of stress arch effect. The influence of reservoir dip and reservoir azimuth wasn’t considered in general model of calculating the stress arch ratio. A new model of calculating stress arching ratio of inclined reservoir was established using the inclusion theory which is used to solve the problem in composite material mechanics. This model can be solved by tensor algorithm. Correlated influence factors were analyzed，such as reservoir dip，strike direction，Possion’s ratio and rock consolidation coefficient. Then the stress arch effect on porosity and permeability change were also analyzed. The result shows that the stress arch effect is obvious in vertical direction for the reservoir with small reservoir dip angle，large aspect ratio，small Possion’s ratio and small rock consolidation coefficient. While，it is significant in horizontal direction for the reservoir with large reservoir dip angle，small aspect ratio，small Possion’s ratio and small rock consolidation coefficient. Average stress arching ratio are only related to rock consolidation coefficient and reservoir Possion’s ratio.The stress arch ratio is usually positive，while it is negative in principal crustal stress direction due to stress concentration especially for inclined reservoir. The effective stress is relative to the stress arching ratio，while stress sensitivity is influenced by the effective stress. When vertical，minimum horizontal，maximum horizontal and average arching ratios of the reservoir are 0.107 2，0.674 7，0.718 1 and 0.5 respectively，the porosity and permeability calculated by the inclusion theory increase by 32% and 76.7% respectively compared with the values calculated by conventional deformation theory for the pressure drop of 30 MPa. The stress arch effect reduces the degree of stress sensitivity of the permeability and porosity in actual reservoir，which cannot be neglected during the development of low permeability tight oil reservoir.