An all fiber high peak power nanosecond pulsed source based on large mode area (LMA) fiber cascade amplification configuration is presented. 10 ns/100 nJ arbitrarily shaped pulses generated from a single mode fiber (SMF) front end are injected into a LMA fiber cascade amplifier. The amplifier is composed of LMA Yb doped fibers with core diameters of 15, 30 and 140 μm. By cascade amplification, shaped nanosecond pulse with 3.2 mJ pulse energy and 0.3 MW peak-power is obtained. The maximum output capacity of the fiber system is further studied. By injecting Gaussian pulse into the fiber amplifier, high energy of 10 ns/7.2 mJ/0.7 MW output and high-peak-power of 1.16 ns/3.7 mJ/3.2 MW output are obtained.