In order to obtain precise center of asymmetrical and semicircular laser spot,a new method for semicircular laser spot center detection based on circle fitting was proposed.Threshold of laser spot image was segmented by morphology algorithm.Rough laser spot edge was detected in horizontal and vertical direction.Short arcs and isolated edges were deleted by contour growing.The best circle edge was acquired by iterative fitting and the final standard circle was fitted by the best circle edge.The result shows that the method is more accuracy and more automatic than method of gravity model being used in traditional large-scale laser facility of automatic alignment system.The accuracy of method to achieve asymmetrical and semicircular laser spot center meets the requirements of the system.