Abstract: In recent years, the volume method is widely used in the Xihu Sag of the East China Sea for self-assessment of newly increased reserves and the reserves found in early development stages. The results are different from different researchers due to the difference in key parameters adopted, such as reserve classification, well controlled oil-gas area, net pay etc., used for the calculation. Based on the exploration and development practice in the Xihu Sag, the determination of key parameters of SEC volume method is carefully discussed in this paper. The results suggest that some basic principles must be followed in using SEC volume method, such as reasonable certainty, data reliability, reasonable planning and a development trial over 5 years. In the classification table of reserves, low, medium and high values corresponding to 1P、2P、3P reserves should be selected according to the reliability of parameters such as net pay, effective porosity, saturation and recovery factor, etc. Based on the data from well testing, and the reversely calculated final recoverable reserves, average reserve/production ratio and other technical parameters, the control radius of a gas well for the main producing layers in the Xihu Sag is determined as 420～740 m, while the control radius for oil wells varies in the range of 330～660 m. When determining the net pay of a unit by isoline area trade-off method, full considerations should be given to the same layer of gas (oil) and water, the maximum thickness of oil-gas layer, and the distribution patterns of structures and sand bodies. With the successful application of the method to gas field N-1 in the Xihu Sag, the research results have certain significance for the reserve evaluation of newly added reserve or the oil and gas fields in early development stages.