Abstract: Installation of flexible lines, such as pipe, cable and umbilical, is an important part of offshore operation for exploring oil and gas, and the key issue is how to assess the installation parameters accurately and determine the weather window. A simplified model and the corresponding analysis method for normal laying of flexible line are presented based on lumped mass method with the considerations for the modeling, the input data of the analysis, the acceptance criteria of the operation and the important parameters used in the operation. Taking a cable installation project as an example, the sensitivity analysis for the layback, the analysis of the axial compression, and the determination of the maximum environmental condition for the operation are performed. The proper weather window is 2.5 m significant wave height and 30° wave direction. The results show that selecting a proper layback is good for offshore operation; it is suggested to avoid the axial compression; for laying cable in shallow water, minimum bending radius (MBR) is the main limiting criterion to decide the maximum allowable environment condition.