Model tests of the suction anchors with taut mooring systems are conducted under combined average and cyclic loads at the optimal loading point on the side wall of anchors. The deformation instability process of suction anchor subjected to constant and variable amplitude cyclic loads are investigated. It is found that too large cyclic accumulative displacement is the main reason for the failure of anchor. The cyclic accumulative displacement of anchor in the horizontal direction is larger than that in the vertical direction due to application of the vertical additional loads

so the horizontal failure of suction anchors occurs for the constant amplitude cyclic loading tests. The accumulative displacement increases faster with higher cyclic load level at a given average load level

and the number of cycles to failure becomes less. The cyclic displacement does not change obviously with the increase of the number of cycles

but increases with the increase of cyclic load level. The cyclic accumulative displacement and cyclic displacement in each direction at the mooring point increase with the increase of cyclic load level for variable amplitude cyclic loading tests. The accumulative displacements are larger in vertical than in horizontal under different cyclic loading time histories

so the moderate preferring vertical failures of anchors occur. Besides

the previous cyclic loading histories have an obvious influence on the accumulative deformation produced by subsequent loading. Compared with the static loading

the movement direction angle of the anchor under cyclic loading increases

which may be due to the greater accumulation of pore pressure at the bottom of the anchor than that at the side wall. From the view of effective stress

the decrease of effective stress at the bottom is relative obvious than that at the side wall

thus vertical bearing capacity decreases more remarkably

and vertical movement of anchor is more obvious.