To fulfill the requirements of the high energy laser beam online sampling diagnosis, an optical sampler based on circular reflector is developed. The optical sampler is circularly shaped, expending the beam circularly, which enables more than half of the detectors to receive the sampling beam. The reflecting surface is arc-shaped, expending the beam perpendicularly, which simplifies the alignment, while increasing the attenuation factor. Appropriate variation ranges of certain parameters are specified through numerical simulation, satisfying the requests of the coupling aperture angle between the detector and the reflected laser beam. Experiments show that this optical sampler can be used in the online diagnosis of high energy lasers with large beam profiles.