Abstract: The instability of pipelines directly installed on the seabed is a “fluid-pipe-soil”coupling problem .In this paper , the instability process of pipelines has been investigated for different submerged weight of pipeline , pipeline diameter and loading history . Based on the presently available test data , a design equation is established , relating the critical Fr number inducing pipeline instability to dimensionless pipeline weight parameter .It is also found that different loading history affects pipeline stability with different degree.