Resistant starches from potato (RS3) were modified by acid hydrolysis combined with heat-moisture treatment (AH-HMT). The effects of changing the starch slurry concentration, the type of acid and its concentration on the structure of RS3 were also studied. The results showed that after AH-HMT, the X-ray pattern remained practically the same with only small changes in the average degree of polymerization (DP), specific surface area (SSA), microstructure (MS), and relative crystallinity (RC). Compared with untreated RS3, the decrease in DP and SSA and the increase in RC caused by high concentrations of acid were significantly greater than those caused by low concentrations of acid. However, the changes in the surface microstructure of RS3 were much more complex. The influence of the five types of organic acid studied on the structural properties of RS3 varied from strong for citric acid, followed by acetic acid, succinic acid, malic acid to weak for lactic acid.