The characteristic and distribution status of space charge directly changes the intensity and distribution of the electric field inside dielectric material and seriously affects the electric performance of electric device. In recent years, nano materials and micronano electric devices develop rapidly, detecting and grasping the characteristic information of space charge in nanoscale even smaller scale becomes a problem to be solved urgently. Based on the basic principle of pulsed electroacoustic method is new space charge distribution test method was designed and implemented based on THz wave and elasticoptic sampling technology. Based on the stressinduced birefringence effect, the elasticoptic sampling sensor was designed and manufactured and its performances are tested. A space charge test system is built and used to test the custom silicon PN junction samples. The results show that the changing rule of space charge region width versus bias voltage is consistent well with the basic electrical characteristics. The method adopts the entire optical technical means, which breaks the limitation of traditional electronic test technology to system bandwidth. Experiment results show that the method designed in this paper can effectively and reliably improve the space charge test resolution to nanoscale.