Biological soil crusts is widely developed in hilly Loess Plateau region，which significantly affects the soil physical，chemical and biological properties of 0-10 cm soil layer. To clarify the comprehensive effect of biological soil crusts of varying developmental ages on soil quality，the soil quality index(SQI)was employed in this study to evaluate the effects of biological soil crusts aged 1 to 30 years on soil quality across different layers，specifically the biological soil crusts layer，namely，0–2，2–5 and 5–10 cm. Principal component and correlation analysis were utilized to identify the key factors of improving soil quality. The results showed that:(1)With the increase of development years，the physical，chemical and biological properties of biological soil crusts layer and subsoil layer were obviously improved. Biological crust decreased soil bulk density，increased soil field water capacity，total porosity and phosphatase content，and increased soil organic matter，total nitrogen，total phosphorus and available phosphorus contents.(2)The improvement of soil quality by biological soil crusts decreased with increasing soil layer depth. Within the same year，the difference between SQI of biological soil crusts layer and 0–2 cm soil layer ranged from 0.11 to 0.30，while the difference between 0–2 and 5–10 cm soil layers was only 0.03 to 0.06.(3)The improvement of soil quality by biological soil crusts was related to the years of development. The SQI of the biological soil crusts reached 0.64 within the first three years of development，and the SQI of 0–2 cm soil layer increased to 0.56 in the first five years of biological soil crusts development，which was 48.82% higher than that of abandoned land. The SQI of biological soil crusts layer and 0–2 cm soil layer reached the highest values around the 13th year，which was 0.85 and 0.67，respectively，and then tended to stabilize.(4)Soil organic matter and total nitrogen content were the key factors that improve soil quality through biological soil crusts. This study clarified the comprehensive effect of biological soil crusts of varying ages on soil quality，and strengthened the understanding of ecological function of biological soil crusts.