Abstract: The ring-cylinder assembly interface of the inertial platform is critical to the accuracy of the inertial navigation system. Its axis pose is affected by coupled multi-source errors, such as geometric tolerances and contact deformation. An assembly accuracy analysis method considering both non-ideal surface topography and contact deformation was proposed for the ring-cylinder interface of an inertial platform. First, the load distribution under preload was obtained through finite element simulation. Then, Zernike polynomials and Legendre–Fourier polynomials were employed to model the non-ideal surfaces of the ring and the cylinder, respectively. A contact mechanics algorithm based on the Conjugate Gradient–Fast Fourier Transform (CG-FFT) was developed to compute the contact interface deformation and fit the axis pose. On this basis, small displacement torsor (SDT) parameters incorporating positioning and orientation errors were introduced as random variables. A parallel Monte Carlo simulation framework was established to perform one thousand simulations. The results showed that the axis position deviations follow a normal distribution. The cylindrical translation parameter contributes 67% to the variance of coaxiality, making it the key factor affecting assembly accuracy. This method reveals the decisive role of positioning accuracy on the axis pose of the ring–cylinder assembly interface, providing a basis for the geometric tolerance design of inertial platforms.