When a multimode fiber transmits a laser beam, the speckle will form in its output field. A dynamic fiber scrambler could be used to suppress the speckle. No matter what actual way of suppressing speckle is, such as fiber scrambling or using a rotating phase plate, the suppression is the result of exerting disturbance in the process of the speckle forming. We could disturb the phase of the input beam with specific method to weaken the speckle effect. To get a speckle image formed by a multimode fiber, we simulate different diffraction patterns under different phase conditions using diffraction model, in which the phase of the input beam is modulated by a rough surface, and then sum them to form the instantaneous speckle. To study the speckle suppression, we superpose instantaneous speckles, and as a consequence the final speckle is suppressed. The simulation would help us understand the speckle suppression experiment with the input

beam phase modulation conducted in our lab.