In order to study the change of the ecological stoichiometric characteristics since returning farmland to grassland with restoration years and its influence on plant diversity, this study examined the changes in the C, N, P stoichiometry characteristics of aboveground part of plant and soil, and explored the relationships between them and the plant diversity with a chronosequence of 8, 15, 25, and 35 years since returing farmland to grassland (farmland as the control) on the Loess Plateau. The results showed that: (1) overall, the content of C, N and P in the aboveground part of plants all increased with the increase of the recovery years, while the C:N, C:P and N:P decreased. The content of plant C reached the maximum value (434.95 g/kg) at 35 years of grassland restoration, the content of plant N reached the maximum value (2.29 g/kg) at the farmland stage, and the content of plant P reached the maximum value (1.23 g/kg) after 25 years of grassland restoration. (2) The content of C, N and P in the soil and N:P showed an increasing trend with the increase of recovery years, while C:N showed a trend of decreasing and C:P remained unchanged basically; all of the maximum values of C, N, P contents appeared in the 35 years, the values are 10.94 g/kg, 0.07 g/kg and 0.06 g/kg, respectively. (3) Shannon Weiner index decreased with the increase of C content and C:N in the aboveground part of plants; the content of C in the aboveground part of plants and soil P are the main factors affecting Simpson index, which showed a negative correlation with C in the aboveground part of plants and a positive correlation with the P content in the soil; Pielou index was positively correlated with the soil N content, N:P, C:P and the C content of the aboveground part of plants; Margalef index was positively correlated with the N and P contents of the aboveground part of plants. The results showed that the C, N, P stoichiometric characteristics of plants and soil had important effect on plant diversity in the grassland restoration.