Distributed generation is increasingly being connected to the distribution network due to its advantages of high flexibility, low cost, green and low carbon. However, because it changes the topology of the original distribution network, it also poses new requirements for traditional current differential protection. Firstly, the impact of the access of a large number of distributed generations on relay protection in the distribution network is analyzed. Then, considering the requirements of relay protection for quickness and sensitivity, the Whale optimization algorithm with fast convergence speed is introduced, and the LM algorithm is introduced to improve it. Finally, the setting value for current protection is optimized when the distributed generation is connected to the distribution network. The Whale optimization algorithm with fast convergence speed is used for global optimization, and to avoid falling into a local optimum, it switches to the LM algorithm for further solution. Experimental analysis results show that the introduction of the Whale optimization algorithm improved by the LM algorithm can accelerate the convergence speed and improve the quickness of the relay protection device, which is suitable for relay protection setting optimization when distributed generation is connected to the distribution network.