The bottom-up and top-down effects in the food web have a significant impact on community dynamics and ecosystem functions. This study aims to explore the effects of mutualism relationships and plant diversity on different trophic levels of food webs in arthropod communities. Three levels of ant-lac insects mutualism treatments (presence, absence of mutualism, and natural control) and three kinds of plant diversity treatments (monoculture, two species planting, and three species planting) were set up by randomized split-plot experiment design. We collected all arthropods on the host plants under by using hand-collection, shake-down, and sweeping method in August and September 2016. Then we identified all arthropods on the basis of different trophic levels. Structural equation modeling was used to compare the path and intensity of the interactions among different trophic levels. The results showed that:1) Mutualism relationship had a significant top-down effect on predators and consumers. The path strength of mutualism ants to predators was stronger than that of natural control treatment. Mutualism relationship had a greater impact on predators than consumers. 2) Plant diversity had a significant bottom-up effect on consumers and predators by affecting plant biomass, and this effect was significantly reduced as the trophic level increased. 3) Consumers were mainly affected by the bottom-up effects of plant diversity, while predators were mainly affected by the top-down effects of mutualism ants. The food web structure with mutualism relationship was more complex, and the interaction between trophic levels was more significant. This study not only explored the effect of mutualism and plant diversity on different trophic levels in arthropod food webs in a lac plantation ecosystem, but also revealed the pathways and strengths of top-down and bottom-up effects to each trophic level. The results have certain theoretical reference value.