The invention provides an on-line measuring device and method for gamma radiation optical glass transmittance. The on-line measuring device for gamma radiation optical glass transmittance comprises ashell, a light source emitter, an optical isolator, a mode matching lens, a first high reflection mirror, a first adjustable diaphragm, a Dove prism, a second adjustable diaphragm, a second high reflection mirror, a focusing lens, a photodetector, a driving control module and an acquisition and processing module, wherein the light source emitter is disposed in the shell and is used for emitting adetection light source; the optical isolator, the mode matching lens and the first high reflection mirror are arranged in the shell; the first adjustable diaphragm is arranged on the shell; the Dove prism is made of optical glass to be measured and is arranged in the irradiation area; the second adjustable diaphragm is arranged on the shell; the second high reflection mirror and the focusing lensare arranged in the shell; the photodetector is disposed in the shell and is used for receiving transmitted light passing through the focusing lens; the driving control module is separately connectedwith the light source emitter and the photodetector and is used for providing driving current for the light source emitter and the photodetector and controlling the output wavelength of the light source emitter; and the acquisition and processing module is connected with the photodetector and is used for acquiring and processing transmitted light signals received by the photodetector.