Abstract: To calculate in-situ stress is the base of the evaluation of sidewall statility of borehole,the forecast of sand production,the casing damage and distortion,the perforation desigh optimization and the design of hydraulic fracturing.To the offshore oil and gas well,the vertical stress is the result of the pressure of seawater,the pressure of formation without logging data and the pressure of formation with logging data.The pressure of formation without logging data has been calculated through the analysis of density along with the depth change trend.Basing on fractureing calculation model with pore elasticity theory,the elastic strain in horizontal direction has been determined with hydraulic fractureing curve.Basing on elasticity combination empirical model of in-situ stress,the in-situ stress section on the oil well in the South China Sea has been calculated with logging data.The result provided useful basic data for the forecast of sand production,the casing damage and distortion,the perforation design optimization and the design of hydraulic fracturing.