In order to explore the causes of fish tenderness changes during microwave cooking of boiled fish, the physical property analysis method can be used to characterize the maximum shear force of fish meat, and it can also be analyzed from the perspectives of water state, protein denaturation and microstructure by method of measurement like Low field nuclear magnetic resonance(LF -NMR). The results showed that at the first stage of microwave heating (150 s), the connective tissue in fish meat was rapidly broken, and the sarcoplasmic protein was almost denatured completely. Part of the bound water was converted into immobilized water, which gave the fish its unique tenderness. At the second stage of heating period (300 s), fish meat tenderness reaches its maximum and is similar to that of traditional cooking. Excessive heating time (390 s) may result in rupture, contraction, reconnection, and stiffening of fish meat muscle. Therefore, the design of one-step microwave method for cooking fish can shorten the time of microwave treatment, which is conducive to maintaining the combination state of water and the integrity of tissues, so as to ensure the tenderness of fish.