As a crucial downhole flow control tool, the O-ring contact width of the downhole intelligent sliding sleeve is a key parameter for assessing the sealing performance of the tool. This paper establishes a contact width prediction model for the O-ring seal in petroleum downhole applications. Based on the Hertz contact theory, the Mooney-Rivlin hyperelastic theory is introduced to characterize the nonlinear mechanical properties of rubber materials, and the expression for contact width is derived. The ANSYS finite element analysis results show that the average error of the established contact width prediction model is 5.9%, with a minimum error of only 1.6% and a maximum error of 10.9%. This provides theoretical support and data reference for the structural design, performance optimization, and engineering application of O-ring rubber seals in petroleum downhole applications.