Abstract: Radio-luminescence from radioactive isotopes has important applications in special fields. To accurately measure the luminescence characteristics of luminescent materials in specific light-emitting directions under radioactive source excitation, a measurement system for transmissive light emission of luminescent materials excited by radioactive sources was constructed. Based on this system, a method for obtaining photometric parameters of weak light generated by bulk samples under radioactive source excitation was established. In addition to the spectrum of transmissive light emission, the photometric parameters include luminous flux, photon number, and radiant flux, with good consistency in multiple repeated measurements. Using this testing system, transmissive light emission tests and comparative analyses were conducted on rare-earth-doped garnet-based ceramic samples with different compositions and surface roughness states under strontium-90 source excitation. The results show that Luminescence Ce:YAG transparent ceramics excited by strontium-90 source exhibit obvious concentration quenching effects. The luminescence of Ce and different element co-doped alumina transparent ceramics excited by strontium-90 source varies with the substitution of Y or Al atoms by co-doped atoms. For 0.5 mm thick Ce:LuAG ceramics, the luminous flux and optical power of transmissive light emission under strontium-90 source excitation decrease in the order of double-sided grinding > single-sided grinding > double-sided polishing. In addition, tests and comparative analyses were performed on the transmissive light emission of CsI(Tl) crystals irradiated by accelerator electron beams for different durations under strontium-90 source excitation. The results show that the luminescence of CsI(Tl) crystals in transmissive light emission excited by strontium-90 source decreases with irradiation. The luminescence attenuation after irradiation first occurs in the 450–490 nm range; with the increase of irradiation time, the attenuation range extends to 450–600 nm, while the luminescence in the 600–700 nm range is not significantly affected by radiation. This work well support studies on optimization of luminescent materials and radiation damage under radioactive sources. However, it is still necessary to improve the suppression of stray light in this test system.