Extracting the equivalent parameters of the weak-coupling and strong-coupling fishnet structure metamaterial based on the traditional retrieval algorithm and the improved algorithm of Kramers-Kronig relations are proposed, respectively. A comparative analysis of the effectiveness and applicability of the two algorithms are also included. The theoretical analysis and numerical results show that the traditional retrieval algorithm can retrieve the equivalent parameters of the weak-coupling and strong-coupling cases of electromagnetic metamaterials accurately, but with high computational complexity. While the improved algorithm based on the Kramers-Kronig relations can reduce the computational complexity and extract the equivalent parameters only for the weak-coupling case of electromagnetic metamaterials. However, it is not suitable for the strong-coupling case which may disobey the continuity requirement of the Kramers-Kronig relations. The presented results may extend the equivalent medium theory and provide a theoretical reference for the design of new metamaterials.