Amplified spontaneous emission (ASE) can strongly affect the operation and efficiency of high power laser devices. In this paper, starting from the laser rate equations and in consideration of the temporal and spatial pump rate distributions, the ASE in a multisegment amplifier (MSA) has been studied in detail by using two dimensional ray tracing and Monte Carlo methods. Numerical calculations have shown that gain distributions at the horizontal section of the middle slab are dependent on the time and pump