The cleanliness of neodymium glass surface in the slab amplifier is a key indicator of guaranteeing high performance and long lifetime of amplifiers, and the reasonable internal flow field distribution is the prerequisite for keeping clean in the amplifier cavity. By means of computational fluid dynamics method and with the Fluent software, the internal flow field of slab amplifier cavity in the purging process of pure nitrogen is simulated, and the model validity is verified on the prototype system. By adjusting the structure and location arrangement of air inlets and outlets of the amplifier, the optimal flow field distribution is obtained. The experimental results show that compared with those of the non-optimized amplifier, the optimized amplifier not only takes only half time to obtain the same required cleanliness, but also the cleanliness of neodymium glass surface is higher.