Dextran has emerged as a promising biopolymer carrier for controlled release formulations of pesticides. In this study, pH-sensitive acetalated dextran microparticles (Pyr@Ac-Dex) are prepared to encapsulate and control the release of pyraclostrobin (Pyr). In vitro fungicidal activity experiments showed that the prepared Pyr@Ac-Dex particles show comparable fungicidal ability against S. sclerotiorum compared to that of Pyr technical. In a 10-day pot experiment, the control efficacy of the Pyr@Ac-Dex treatment against S. sclerotiorum (77.1%) is significantly higher than that of Pyr emulsifiable concentrate (Pyr EC) treatment (42.4%). Photodegradation experiments show that compared to Pyr technical, Pyr@Ac-Dex doubles the half-life of Pyr in water. Acute toxicity experiments show that Pyr@Ac-Dex significantly reduced the acute exposure toxicity of Pyr to zebrafish. This study provides an environmentally friendly, feasible, and sustainable strategy for plant disease management.