We have experimentally investigated high-energy rectangular pulses delivering from an erbium-doped fiber laser operating in ultra-large net-negative-dispersion regime. The laser oscillator emits rectangular pulses with approximately Gaussian spectral profiles, adjustable nanoseconds durations, and multi-ten nanojoule energies. The outputted pulses can be further amplified to over 2 μJ without distortion by a two-stage erbium-doped fiber amplifier. Thus, rectangular pulses with controllable durations and energies can be achieved from the compact all-fiber fiber laser system.