The cement sheath absence in the horizontal well affects the strength and safety of the casing. To address the problem, considering the cement sheath absence position, thickness and angle, the finite element analysis method was used to establish the horizontal well casing-cement sheath-formation mechanical analysis model. The effect of casing internal pressure and non-uniform geostatic stress, the stress distribution of the casing at the cement sheath absence section in horizontal well was simulated. The influence of the thickness of the cement sheath at the first and the second cementing surface on the safety of the casing strength was analyzed. The analysis results showed that the absence of the first cementing surface of the cement sheath has a great influence on the stress distribution of the casing. There is a critical value in the absence ratio, at which abrupt casing stress occurs when the cement sheath absence increases. Under the second cementing surface absence, the casing stress gradually increases with the thickness of cement sheath absence, but without sudden stress change. The position of the casing stress extreme value was observed at the absence angle of 60°.