Abstract: Base on the reanalysis data of the National Centers for Environmental Prediction (NCEP), the strength variations of the double typhoons of "Nesat" (1709) and "Haitang" (1710) after successively landing in Fujian province in 2017 were diagnosed and comparatively analyzed.The results show that the strength variation of water vapor transport at low latitudes is the main reason for the strength difference of the double typhoons.The equatorial crossing airflow plays a key role in water vapor transport.The intensity of subtropical westerly jet is consistent with that of the mid-low level wind field, and the rapid enhancement of subtropical westerly jet is accompanied by an enhancement in the mid-low level easterly wind, which are the reason for the strength of "Haitang" typhoon remains unchanged, whereas that of "Nesat" typhoon decreases rapidly.The effect of double typhoons plays an important role in the strength variation.When the double typhoons close to and interact with each other, the residual circulation of the "Nesat" typhoon is involved in the circulation of the "Haitang" typhoon, which transports a positive vorticity factor and baroclinic energy to "Haitang" typhoon.After the typhoon' landing, the value of vertical wind shear of three different layers shows a consistent decreasing trend.When it is below 6 m·s^-1, it is favorable for the long-term maintenance on land for the weak typhoon "Haitang".The coastal Frontogenesis and thermodynamic factors of sea surface flux play an important role in developing and maintaining the meso- and small-scale convection along Fujian and Guangdong provincial coasts, which aggravates the asymmetric structure of "Haitang" typhoon, and consequently promotes the strength of "Haitang" typhoon to remain unchanged.