Abstract: Bremsstrahlung is the main mechanism of energy loss of space plasma when the magnetic field in plasma is weak. The results calculated at 1 AU of Bremsstrahlung radiation show that the variation of emission intensity I_f has direct relation to the frequency of electromagnetic wave which has been radiated from the plasma. The emission intensity will increase sharply when the emission frequency is close to plasma frequency. The emission intensity will logarithmically decrease slowly when the emission frequency increase. The brightness temperature T_b is proportional to the electron temperature T_e and optical depth of the medium. The variation tendency of T_b is consistent with that of I_f. The emission intensity and brightness temperature in quiet sun period are lower than that of in solar flare at the same emission frequency.