In this paper, transient gain of a quantum-dot semiconductor optical amplifier (QD-SOA) is studied. Waveguide of the QD-SOA is considered to have a tapered structure in which width of the waveguide increases along the QD-SOA. It is observed that by employing tapered waveguide, gain as the key feature of the device acquires more stability, investigated by studying the impact of a powerful optical pulse on the gain as it passes through the amplifier. Thus, by gradually increasing the width of the waveguide along QD-SOA active region, drop in the gain, caused by the strong pulse, decreases. Transient gain of the device is obtained for several outputs to input width ratios. It is demonstrated that as the width ratio increases, gain stability improves drastically; as for width ratio of 10, stability increases over 10 times compared with the generic QD-SOA. In addition to the gain, cross-gain modulation as a nonlinear process, which depends on the gain instability imposed by strong pulses, is studied. In this paper, the rate equations are employed for modeling tapered waveguide QD-SOA. MATLAB ODE (MathWorks, MA, USA) along with the finite difference method is used for studying and simulating the device. Copyright © 2013 John Wiley & Sons, Ltd.