Using spherical wave beam to illuminate an axicon will result in a beam with small central spot and large focal depth, which is useful for laser communication and long-distance measurement. It is possible that the incident spherical wave beam is tilted with the optical axis of the axicon in practical measurement. Based on the Kirchholf diffraction theory and stationary phase method, the diffraction characteristic of an axicon for a tilted spherical wave beam is derived, and the radial energy distribution is also analyzed. Experiment and simulation are performed with an axicon with 15 mm radius and 1° basal angle. The experimental results are compared with the theoretical analysis and computer simulation. Simulation and experiment indicate that decreasing incident spherical wave′s radii of curvature, propagation distance and the tilting angle can weaken the tilting effect on image of diffraction.