Introduction 'Saiwaihong' apple (Malus pumila 'Saiwaihong') is a premium cultivar cultivated in the Horqin sandy land of Inner Mongolia, where soils are characterized by low organic matter content and poor nutrient retention capacity. Castor bean enzyme biofertilizer (CB), produced through microbial fermentation of castor meal, has shown promise in enhancing crop productivity, but its effects on fruit tree performance and underlying mechanisms remain inadequately characterized.Methods A two-year field experiment was conducted using 7-year-old 'Saiwaihong' apple trees in a randomized complete block design with six treatments: unfertilized control (CK), cattle manure (N), compound fertilizer (F), and three CB application rates. Soil physicochemical properties, enzyme activities, fruit yield components, and quality parameters were systematically evaluated.Results In this study showed that CB application uniquely reduced soil pH from strongly alkaline toward near-neutral conditions and elevated soil organic matter by 50.9-70.6% above the control. Available phosphorus increased dramatically under B1 and B2, accompanied by sustained enhancement of hydrolytic enzyme activities and suppression of polyphenol oxidase. The B2 treatment achieved superior yield enhancement through increased individual fruit size. Most notably, CB treatments elevated fruit vitamin C content by 101.4-135.9% across both years-an effect size substantially exceeding reported values for conventional amendments. Strong positive correlations between soil enzyme activities and fruit quality parameters indicated functional coupling between enhanced soil biological functioning and improved nutritional quality. Our findings demonstrate that moderate CB application (21, 500 kg & centerdot;ha-1) optimally enhanced soil properties, enzymatic activity, yield, and fruit quality in 'Saiwaihong' apple production. These benefits are mediated through improved soil physicochemical conditions and enhanced biological nutrient cycling.Discussion These findings position CB as a promising strategy for building longterm soil health and supporting sustainable orchard management in nutrientpoor sandy soils, offering theoretical guidance for green agriculture development.