In traditional confocal microscopy, there is a tradeoff between the spatial resolution and the field of view due to the limitations of the objective lenses. To solve this problem, diffractive optical elements (DOEs) are used to generate illumination spots with high NA in a large area simultaneously. However, such DOEs in current research are only used as illuminators. In this work, the idea of superposition is utilized in DOEs to have flexible functionality and replace high-NA objectives for confocal measurement. To design the DOEs, different numerical simulation methods for light propagation are investigated and compared. Rayleigh-Sommerfeld integral is chosen to simulate the DOEs to get accurate results. Two kinds of DOEs are designed and simulated for 3D confocal surface measurements.