In the high energy pulsed fiber amplifiers, due to the long gain region of fiber, amplified spontaneous emission (ASE) builted during the space of successive pulses is the most important limitation of the increase of storage energy and extraction capabilities. An analytic model is developed to evaluate the ASE, storage energy, gain and extractable energy from low frequency, high pumped fiber amplifiers for various numerical apertures, overlap integrals, reflection index, core radius, fiber length and pump power. This model could reflect the physics essence compared to numerical model. The model provides a good assessment for the maximum extractable energy and gain from high energy pulsed amplifiers.