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Achieving an all-fiber ultrafast system with above kW average power and mJ pulse energy is extremely challenging. This paper demonstrated a picosecond monolithic master oscillator power amplifier system at a 25 MHz repetition frequency with an average power of ~1.2 kW, a pulse energy of ~48 μJ, and a peak power of ~0.45 MW. The nonlinear effects were suppressed by adopting a dispersion stretched seed pulse (with a narrow-linewidth of 0.052 nm) and a multi-mode master amplifier with an extra-large mode area, then an ultimate narrow bandwidth of 1.32 nm and a moderately broadened pulse of ~107 ps were achieved.Meanwhile, the great spatio-temporal stability was verified experimentally, and no sign of transverse mode instability appeared even at the maximum output power. The system shown great power and energy capability with a sacrificed beam propagation product of 5.28

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