This paper investigates the mix design optimization of the ultra-high performance seawater sea-sand concrete （UHPSSC） based on the simple centroid design method， and the effects of chopped ultra-high molecular weight polyethylene （UHMWPE） fiber and steel fiber on the workability and mechanical properties of UHPSSC were also studied. The experimental results indicate that the best mass fraction of cement， silica ash， and fly ash in the optimal mix design is 75%， 15%， and 10%， respectively， by comprehensively evaluating the flowability， flexural strength， and compressive strength of UHPSSC specimens. With the increase of volume fraction of chopped fibers， the flowability of UHPSSC decreases， while the flexural strength， compressive strength， and flexural toughness increase gradually. The UHMWPE fiber has a greater influence on the flowability of UHPSSC compared with the steel fiber， however， the improvement effect of steel fiber on mechanical properties is more significant. The failure mode of UHPSSC specimens under bending tests changes from brittle failure to ductile failure with the increase of volume fraction of UHMWPE fiber， and the amplitude of the second peak load is greater than the initial crack load when the volume fraction of UHMWPE fiber is 1.0%. Besides， for the specimens with both UHMWPE fiber and steel fiber mixed， the flowability of UHPSSC decreases slightly， while the flexural strength， compressive strength， and flexural toughness significantly improve. This study can provide some reference for the design and engineering application of UHPSSC.