Fast-growingwood is particularly favored as the lightweight material and has potential used in construction and furniture. However, dimensional instability, one of fast-growing wood’s inherent weaknesses, limits its utilization as solid wood products. In this study, fast-growing Ailanthus altissimawood was modified with 1,3-dimethylol-4,5-dihydroxy ethylene urea (DMDHEU) after ultrasound-assisted pretreatment to improve its hygroscopic dimensional stability. Ultrasound was first used in combination with DMDHEU for wood impregnation modification. The weight percentage gain (WPG), absolute dry density, tangential/radial swelling rates, resin distribution and functional groups of chemical components were investigated. The results indicated that the WPG and absolute dry density of DMDHEU-ultrasonic samples increased compared to DMDHEU-untreated samples. The hygroscopic dimensional stability of DMDHEU-ultrasonic samples improved to varying degrees compared to DMDHEU-untreated samples. In particular, the tangential/radial swelling rates of DMDHEU-ultrasonic-4% NaOH samples decreased from 2.01%/1.22% for untreated samples to 0.55%/0.38%. On the one hand, ultrasound-assisted pretreatment affected the distribution of resin inside vessel. Raised-membranous resin appeared along the vessel of DMDHEU-ultrasonic-4% NaOH samples, while only sporadic resin deposition was observed for the DMDHEU-untreated samples. On the other hand, ultrasound-assisted pretreatment promoted better penetration of the DMDHEU resin into the wood cell wall, and the hydroxyl groups in the hydrophilic part were converted into hydrophobic ester bonds and ether bonds. In conclusion, ultrasound-assisted pretreatment had a positive effect on the wood DMDHEU impregnation. And these findings shed light on promising pretreatment methods for wood dimensional stability modification.