A wheat pot experiment was conducted in arid-area loess amended with urban sludge compost, Tessier sequential extraction procedure was used to analyze Cd species distributions in soil, and the effects of sludge compost on Cd contents in different wheat organs and on Cd species in loess–wheat system were studied. The results showed that no significant difference occurred in wheat biomass of the control (without sludge compost), while biomass of wheat roots, stems and leaves in the sludge compost treatments, shells and grains increased firstly and then decreased with the increase of Cd-stress level. Cd contents increased in all wheat organs with the increase of Cd-stress level and in an order of root > stem and leaf > shell > grains. Cd bioavailability increased and Cd potential hazard to wheat system aggravated with the increase of Cd-stress level. The partition coefficients of organic-and residual-Cd increased significantly while that of carbonate Cd decreased, and sludge compost increased the unavailable Cd form. The total redistribution coefficients of Cd in soil with or without sludge compost increased with the increase of Cd-stress level, while bond strength coefficient of Cd increased. Sludge compost decreased total redistribution coefficient of Cd while increased bonding strength coefficient of Cd, which suggest that sludge compost inhibited Cd activity in soil. The exchangeable Cd contributed most to Cd contents in all wheat organs.