A precise detection technique with array camera is discussed for the detection of dynamic focus position in this paper. This technique is well applicable for measuring the focal spot shifting of large and slow lens or optical systems. Two coated wedges are adopted in light path to supply bands of beams, which form the spots matrix on the detector. Focus position shifting can be indirectly obtained from position shifting of the smallest spot on the detector. The technology discussed here is simple and accurate; it also solves the problem that dynamical focal spot position shifting is immeasurable for conventional focal length measureing methods.