As a quinazolinocarboline alkaloid, dehydroevodiamine (DHED) derived from Evodia rutaecarpa has recently gained attention because it improves memory failure in Alzheimer's mice. However, its solid forms, which have not yet been elucidated, could be a vital factor affecting its bioactivity and dosage form design. In this study, five DHED-related crystals, including DHED, its hydrochloride anhydrate (DHED·HCl), methanol solvate (DHED·HCl·CH₃OH), dihydrate (DHED·HCl·2H₂O) and trihydrate (DHED·HCl·3H₂O), were obtained via re-crystallization. The five crystals fell into four distinct structural types based on the analysis of their similarities, and DHED·HCl·CH₃OH and DHED·HCl·2H₂O were proven to be isostructural. The water or solvent molecules incorporated into the crystal lattices via hydrogen bonds and π–π interactions were the main causes of formation of the distinct solid DHED hydrochloride forms. After dehydration at a high temperature, DHED·HCl·3H₂O was transformed into an amorphous form (DHED·HCl-AM). Stability tests showed that the two isostructural crystals exhibited the same solid forms under all investigated conditions, and the humidity-induced phase transformation of DHED·HCl was closely tied to the moisture content. Solubility experiments showed that DHED achieved the highest solubility of approximately 3.6 μg ml⁻¹, because of the loose molecular arrangements in DHED with the smallest packing coefficient and lack of π–π interactions. Additionally, transformations among evodiamine (EVO), DHED, DHED·HCl·2H₂O and rutaecarpine (RUT) were also revealed: EVO was transformed into DHED·HCl·2H₂O in hydrochloric acid-acidified acetone, and all of the solid forms of DHED hydrochloride were converted to RUT at 523 K. The transformations provided a novel and reliable approach to prepare these compounds.