With the development of fishery economy, fishing gear materials, such as cotton, which are generally biodegradable, have been replaced by synthetic materials, such as polyethylene, polypropylene, and polyamide, which are difficult to be degraded. Moreover, fishing nets made from synthetic fibers, are not able to degrade in marine environment for decades. When they are lost or abandoned at sea, they will continue to trap fish and other animals, becoming "ghost fishing" gears. In face of the threats of "white pollution" and "ghost fishing" resulting from the use of non-degradable fishing equipment, the development of biodegradable fishing materials has become one of an effective path towards the sustainable development of the fisheries. Starch is widely present in nature, renewable, and can be used as a biodegradable material. However, due to poor processing and mechanical properties, they are often blended with other polymer materials. It is noted that the purpose of accelerating or slowing down the degradation of composite materials can be achieved by adding nanomaterials to modify biodegradable materials. Based on this, the starch (STR)/high-density polyethylene (HDPE)/montmorillonite (MMT) nanocomposite fibers were prepared by melt-spinning method, and the effect of nano-MMT on the thermal properties, mechanical properties, dynamic mechanical properties and degradation behavior under seawater of nanocomposite fibers was studied. The results showed that the melting temperature (T_m) of the nanocomposite fibers was reduced, the crystallinity of the nanocomposite fibers increased, and their breaking strength decreased as well. What’s more, the addition of MMT significantly reduced the glassy storage modulus and internal loss of polyethylene, but it had little effect on the starch. After 4 months of seawater degradation, compared with that of STR/HDPE fibers, the weight loss rates of STR/HDPE/MMT nanocomposite fibers increased by about 5%, and the loss of fiber diameter increased by about 11%. This indicated that MMT accelerated the degradation process of STR/HDPE fibers. This paper systematically studies the seawater degradation properties of the MMT modified starch-based nanocomposite fibers, which can provide reference for the development and application of biodegradable materials for marine industries such as fisheries.