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FY-4 satellite cloud images, the hourly surface AWS data, NCEP-FNL reanalysis data and ERA-Interim SST data are used for analyzing intensity characteristics of Typhoon Yagi (1814) and typhoon Lekima (1909) above the Bohai Sea. The results show that Yagi’s intensification was accompanied by the invasion of cold air in the middle and upper layers. When the cold air invaded the centre of the typhoon cloud, the intensity of typhoon Yagi weakened. Before the typhoon Lekima entered the sea, the cold air had intruded into the typhoon’s centre; the typhoon weakened and died after entering the ocean. The warm-core structure of the northward typhoon was destroyed, and the symmetrical structure became asymmetrical. The lower level of the typhoon showed baroclinic characteristics. Typhoon Yagi entered a strong divergence zone through the Bohai Sea. However, the Lekima entered a weak convergence field through the Bohai Sea. The high-altitude divergence in the northward direction was conducive to the strengthening of the typhoon before the typhoon lands. The vertical wind shear and the typhoon intensity were distributed opposite, but the position conversed when the typhoons land. After Yagi entered the sea, the water vapour channel was broken, and its strength above the sea was more dependent on thermal and dynamic conditions. The water vapour flux and water vapour flux divergence of Typhoon Lekima originated from its circulation. After Yagi entered the sea, the latent heat heating rate increased sharply. The lower layer of the Lekima maintained a weak latent heating centre until the typhoon disappears.