Abstract: To explore the biological indices of soil fertility and quality evaluation of purple arid hillside field in the Three Gorges Reservoir of China, the contents of soil microbial carbon (SMBC) and soil microbial nitrogen (SMBN) in crop-mulberry agro-forest system in purple arid hillside field in the Three Gorges Reservoir was studied. The study used the arid hillside crop field protected by mulberry as a typical soil and water conservation mode to determine the biological indices of soil fertility and quality. The experiment consisted of four treatments, which were CT treatment (crops were planted along transverse slope), T1 treatment (crops were planted along transverse slope with mulberry trees on upper-slope and lower-slope), T2 treatment (crops were planted along transverse slope with mulberry trees on upper-slope, middle-slope and lower-slope); T3 treatment (crops was planted along transverse slope with mulberry trees on middle-slope and the four sides of the slope. The results showed that slope length and layout of mulberry caused significant differences in SMBC and SMBN contents. The T1 treatment significantly limited the contribution of SMBN to total nitrogen but averagely increased SMBC by 23.43 mg·kg^-1. Also the treatment T3 significantly reduced microbial quotient and averagely increased SMBN by 21.81 mg·kg^-1. Furthermore, soil microbial biomass was highly significantly correlated with soil carbon and nitrogen in treatment CT. However, SMBC and SMBN in different layouts of mulberry were largely not significantly correlated with soil carbon and nitrogen in purple arid hillside field crop-mulberry system. Therefore while hillside field crop-mulberry modes increased soil microbial biomass to some extent, the viability between SMBN and SMBC in purple arid hillside field crop-mulberry system in the Three Gorges Reservoir was not consistent. The study showed that both SMBN and SMBC were datable evaluation indices of soil quality in purple arid hillside fields.