Abstract: Offshore wind power is an important resource in China’s thirteenth five-year plan of renewable energy development. Compared with near-shore wind farms, offshore wind farms have more complicated wind conditions, higher construction costs and lower risk bearing capacity. It is necessary to carry out long term wind energy resource assessment, which can provide offshore wind farms’ feasibility study and risk assessment. The NCEP (National Centers for Environmental Prediction)’s CFSR (Climate Forecast System Reanalysis) data have advantages of global coverage, high resolution and large marine data sources. In this study, the NCEP and CFSR data were applied to access the long term wind energy resource of the Xiangshui offshore wind farm in Jiangsu province. The results showed: (1) the mean deviation and root-mean-square-error of the comparisons between CFSR and CFSR 10m height wind speed were -0.2 m·s-1 and 1.9 m·s-1, respectively.(2) The correlation coefficients between CFSR data’s 10m height wind speed and the offshore mast data’s 90 m height wind speed were 0.359-0.619, which were improved by 40%~90% compared to the meteorological station’s correlation coefficients. CFSR data could improve the long term correction method’s accuracy and correction result’s reliability. (3) The recent 30-year wind data of the offshore mast’s 90 m height were obtained by application of CFSR data to long term correction, with the multi-annual mean wind speed of 6.9  m·s-1 and the standard deviation of 0.2  m·s-1 (2.9%). (4) The recent 30-years equivalent full load hours of the offshore wind farm were calculated by the wind energy resource assessment software WAsP, with the multi-year mean value of 2 423 h and the standard deviation of 112 h (4.6%). (5) By the evaluation by the normal distribution model, the minimum yearly mean wind speeds in the 25 (the lifetime of offshore wind farms), 10 and 5 years period were 6.6, 6.7, and 6.8 m·s-1, respectively; and the minimum equivalent full load hours were 2 227, 2 280, and 2 329 h, respectively. In conclusion, it is reasonable that applying the CFSR data on the offshore wind farms’ long term wind energy resource assessment.