Nano-hydroxyapatite reinforced poly(vinyl alcohol) (nano-HA/PVA) gel composites has been proposed as a promising biomaterial to replace diseased or damaged articular cartilage. In this paper, the stress relaxation mechanism of nano-HA/PVA gel composites was investigated. The various influence factors on the stress relaxation behavior of the composites were also evaluated. The results showed that the relaxation mechanism of the composites was mainly determined by the synergistic effect of two stress relaxation mechanisms analogous to those of the natural articular cartilage and the polymer. The relaxation rate of the composites increased with the rise of strain ratio, but it declined with the relaxation time. Under the given strain ratio, the relaxation rate of the composite presented a trend of rising first and then falling with the increasing amount of nano-HA content. Contrarily, the normalized equilibrium relaxation modulus of the composites decreased first and then presented increasing trend with the rise of nano-HA content. Furthermore, the normalized equilibrium relaxation modulus of the composites decreased with the rise of strain.