Abstract: Since the central spot diameter of exit beam generated by the spherical wave beam passing through a conical lens is small and the focal depth of the exit beam is long, it is suitable for long distance measurement and alignment in laser communication. The diffraction mode of the conical lens for the spherical wave beam with eccentricity was emphatically investigated to reduce the effects of the off-axis phenomenon on measurement, and to prevent the center-line deviation or tilt of the spherical wave beam. Based on the diffraction theory and stationary phase method, the diffraction mode of the conical lens for the spherical wave beam with eccentricity was derived, and the characteristic of its radial energy distribution was analyzed. The experiments and simulations are performed by using a conical lens whose radius is 15 mm and basic angle is 1°. The experimental results are compared with the theoretical analysis and computer simulation results. The comparison indicates that the influence of eccentricity on the diffraction mode decreases as the radius of curvature of incidental spherical wave and the propagation distance decrease. Therefore, the spherical wave with small radius of curvature instead of plane wave can extend the focal depth and reduce the influence of eccentricity on the measurement.