To measure output parameters of a laser amplifier, with the repetition rate of several tens of minutes one pulse and single pulse energy of 5 J, a special designed measurement system is put forward for high-energy, large cross-section, low frequency laser system. This system uses the principle of Fourier image transfer to realize real-time and clear monitoring to the output characteristics of the laser beam. And this system can not noly detect laser wavelength, spectral width, spot size, single pulse energy, repetition rate, and polarization direction of the laser beam simultaneously, but also statistically calculate the beam modulation, divergence angle, directivity, beam quality factor, energy stability, average power, power stability, pulse width, pulse stability, and degree of polarization.