Dialdehyde starch has been used to improve comprehensive properties of thermoplastic starch (TPS) in this study. Dialdehyde sweet potato starch (DASS) with various carbonyl contents has been prepared using sodium periodate to oxidize sweet potato starch in the neutral condition. The structure properties of DASS were analyzed by SEM and XRD. The results demonstrate that DASS with the higher carbonyl content comprise lower crystallization. The thermal, mechanical and biodegradable properties of TPDASS were investigated, and the results illustrate that the moisture adsorption and biodegradable ratio decreases with the Tg of TPDASS with higher carbonyl content. This phenomenon could be attributed to the partial cross-linking of DASS which is due to semi-acetal formation by itself. Meanwhile, the mechanical properties of TPDASS95 were less sensitive to the content of plasticizer. The tensile strength could reach even 12.9 MPa while the water content was higher than 13.5% and the biodegradable ratio was 28%.