Abstract: To explore the changes of starch in pyroconversion process, pyroconversion dextrin (PD) was prepared from tapioca starch (TS) under the catalysis of hydrochloric acid by dry method. The optimal preparation process of PD was obtained by single factor and orthogonal test based on the indicators of dietary fiber content and whiteness. The properties including morphology, solubility, crystallinity, and acid-thermal stability of PD were determined. The results showed that the dietary fiber content of PD was 60.79%, whiteness was 45.27, and solubility in cold water was 93.81% at the pyroconversion temperature of 170℃, the pyroconversion time of 150 min and the addition amount of hydrochloric acid of 14%. Compared with the solubility of native starch, the cold-water-solubility of PD was significantly improved. Microscopy characterization showed that PD from TS performed a yellow color and its granule morphology remained intact in 100% glycerol. In addition, the birefringence of PD was weaker than that of TS. The relative crystallinity decreased from 29.41% (TS) to 9.48% (PD). The results of X-ray diffraction revealed that crystal structure of TS was destroyed by pyroconversion. The acid-thermal stability of PD was reflected by the change of reducing sugar content. The increase of reducing sugar content was less than 1% when PD was stored (25℃/24 h) at pH 1-7 and heated (85℃/0.5 h) at pH 2-7.