Direct shear tests were carried out and the shear strength,cohesion and internal friction angle of palm fiber reinforced clay mixed with nano-SiO₂ were investigated.The results show that the palm fiber can significantly improve the shear strength and cohesion of clay,but has little effect on the internal friction angle,and the optimal mass reinforcement precent of palm fiber is 1.0~1.2%.With the mix of nano-SiO₂ into palm fiber reinforced clay,the shear strength is further improved,and the cohesive force and the internal friction angle are also improved obviously.For the clay with the same mass reinforcment precent of palm fiber,the shear strength achieves the largest when the mass percent of nano-SiO₂ is 1.0%.For the reinforced clay with the mass reinforcment precent between 1.0%~1.2% of palm fiber,the shear strength is the largest when the mass percent of nano-SiO₂ is 0.7%~1.0%.Further,the action mechanisms of palm fiber and nano-SiO₂ were analysed.The micro structure of the palm fiber reinforcement clay with nano-SiO₂ is more compact,the clay becomes more stable and its shear strength is higher.So nano-SiO₂ can play a role in the reinforcement of subgrade soil.