The invention relates to a device and a method for detecting weak absorption of an optical element. The device comprises a detection laser device, a photoelectric receiver, and a first light splitting element, a reflector and a second light splitting element which are arranged between the detection laser device and the photoelectric receiver, wherein a parallelogram is surrounded by the first light splitting element, the reflector, the second light splitting element and the optical element, and the first light splitting element, the reflector, the second light splitting element and the optical element are arranged at the peaks of the parallelogram. The device further comprises an excitation structure for producing excitation light; the surface of the optical element is opposite to the excitation structure. Compared with a surface thermal lens technology in the prior art, the device disclosed by the invention has the advantages that the deformation of the optical element is extremely small under heat action, so that a light change signal is weak, and available light signals are covered by noise; an interference pattern generated between a second laser beam and a third laser beam is more obvious, the weak absorption of the optical element is analyzed by the interference pattern, so that the detection sensitivity is higher; moreover, the weak absorption device for the optical element is simpler in structure and high in performance-to-cost ratio.