High hydrostatic pressure (HHP) treatment was combined with octenyl succinic anhydride (OSA) modification of quinoa starch (QS) to improve esterification efficiency. The modified QS was used as a stabilizer to prepare a Pickering emulsion. The results showed that the HHP treatment disrupted the morphology and crystalline structure of QS, exposed numerous hydrophilic hydroxyl groups, and added esterification reaction sites. The degree of substitution (DS) and esterification efficiency (RE) of OSA-HHP-QS were significantly (p < 0.05) increased compared with OSA-QS. In addition, the short-range ordering, relative crystallinity, and thermal stability of OSA-HHP-QS decreased with increasing treatment pressure. Except for OSA-HHP200-QS, the starch granules treated at 200 MPa were annealed, resulting in molecular recrystallization. The Contact angle, emulsion stability index (ESI), and emulsion activity index (EAI) indicated that the emulsions stabilized with OSA-HHP-QS were highly stable. Therefore, HHP can be used as a novel technology to assist OSA modified starches in stabilizing Pickering emulsions.