Abstract: The semiconductors refractive indexes change when they are exposed to radiation pulses. For short carrier lifetime, some semiconductors refractive indexes recover quickly. This process can be used to develop a new kind of pulse radiation detector. An interferometer was established to sense the change of refractive index of 300 um GaAs plate. The probe beam was provided by infrared (1310 nm) single mode continuous wave laser. The infrared beam illuminated GaAs with a certain angle. Two main branches of this beam interfered along the reflected path. One branch was reflected from front cover of GaAs. The other branch was the light refracted into GaAs and reflected back from the back cover of GaAs and then refracted out of GaAs. The interference light intensity changed with the change of refractive index of GaAs. The pump source was pulse radiation, 300 keV, 20 ns. The interfered light intensity was recorded by InGaAs infrared 775 kHz detector. Change of refractive index caused by the pulse radiation was detected. The nonequilibrium free carrier density and the change of refractive index were computed about 1014 cm-3 and 10-6, respectively. The experiment results were approximately consistent with the computed results, which suggests that the index modulation technology can be used for pulse radiation detection.