The work aims to study the influence of hydrothermal solution pH value on the composition, morphology, corrosion resistance and cell compatibility of the pearl coating on the AZ91D alloy substrate. The pearl coatings were hydrothermally synthesized on the AZ91D substrate by hydrothermal reaction solutions at different pH values (5.8, 7, 8.8, 11.8). The chemical composition and micromorphology of the pearl coatings were analyzed by XRD, XPS and SEM. The open circuit potential and polarization curves of the materials in simulated body fluid (SBF) were evaluated by an electrochemical measurement system. The cell compatibility of the coatings was examined by cell counting kit-8 (CCK-8 method) and cell fluorescence photographs. With the change of the hydrothermal reaction solution pH value, the composition and morphology of the pearl coatings varied significantly. The electrochemical corrosion test confirmed that the samples deposited with the pearl coating had higher OCP values, the Jcorr values were reduced by nearly 10 times, and the Ecorr values shifted in the positive potential direction compared with the blank control sample. In addition, the coatings deposited under the weak alkaline conditions had the best corrosion resistance, the Ecorr(vs.SCE) value was increased by 0.16 V, and the Jcorr value was reduced by 8.16×10–5 A/cm2. Cytocompatibility test results showed that the pearl coatings were non-toxic, and the large amounts of bone growth factors provided by the pearl powder raw material could promote the proliferation of osteoblasts. The pearl coating is successfully prepared on the AZ91D alloy substrate by hydrothermal reaction method and can effectively improve the corrosion resistance and biocompatibility of the magnesium alloy in the human body.