Abstract: Fresh water is commonly used in pipeline's hydrostatic test, However, for the pipeline installed in the area rare of fresh water and the area using freshwater will increase construction cost, the paper brings forward the necessity for using seawater rationally in coastal regions, Combined with the practice of hydrostatic test with seawater in Libyan Oil Pipeline.it points out that seawater as hydraulic medium shall be modified for water's quality by means of corresponding equipment and meterials. Principles in seawater treatment, treating process, requirements and concerns are introduced in the paper.