This study investigated the synergistic effect of temperature and ultrasonic field on seawater modification of wheat straw fibers via orthogonal design. Based on orthogonal results, physicochemical and thermal properties of wheat straw fibers were also reported. The results indicated that a 120 min seawater modification of the fibers at a 70 °C heating temperature and 90 W ultrasonic power increased water absorption. The increase was attributable to the removal of the waxy layer and non-cellulosic materials, which, in turn, decreased the silicon elemental content and hydrogen bonding, as well as increased surface roughness, crystallinity, and thermal stability. The physicochemical and thermal characterization showed that this modification method has potential to be a viable industrial application.