Montel mirror coated by laterally graded multilayer is a key two-dimensional collimating optics for x-ray analysis instrument. To provide guidance for the fabrication, the collimation property of a W/Si multilayer Montel mirror with parabolic shape is simulated systematically by a ray-tracing program. The influence of different source sizes, multilayer reflection bandwidth, mirror-detector distance, figure errors, and the assembly errors on the collimation performance is simulated. The mechanism for the change of divergence angle, collimation efficiency, beam uniformity, and so on is analyzed. Based on the results, the optimum mirror structure and the tolerance for fabrication and assembly errors were estimated for a high-performance x-ray Montel mirror.