The spontaneous radiation energy of current filament in high gain GaAs photoconductive semiconductor switch (PCSS) is researched. Based on the recombination of non-equilibrium carrier of current filament in GaAs PCSS, the formula of the spontaneous radiation energy of current filament is exactly derived. The theoretical model of the spontaneous radiation of the current filaments in GaAs PCSS is established. Under the condition that the current filament reaches a steady state, the spontaneous radiation energies from the 875 nm radiation and the radiation of four peak wavelengths at one end of current filament are calculated. The maximum optical output energy from the 890 nm radiation is consistent with experimental observations, explaining reasonably the spontaneous radiation phenomena of current filament and lending support for the model′s predictions about the spontaneous radiation energies from the other wavelengths radiation. This lays the foundation for further quantificational analysis on photoionization effect of current filament.