Abstract: Exploring the legacy effects of straw returning on the functional diversity of soil microbial communities provides theoretical support for sustainable agricultural development. In 2016, four treatments were established in Shengli Village, Hailun City, Heilongjiang Province, including no-tillage straw cover (D0), straw uniformly mixed in the 0 ~ 15 cm soil layer (D15), in the 0 ~ 35 cm soil layer (D35), and in the 0 ~ 50 cm soil layer (D50). After 2017, straw return was discontinued, and the impact of different straw return methods four years later on the functional diversity of soil microbial communities in black soil was investigated. The results revealed that the average well color development (AWCD) of soil microbes under the legacy effect of straw returning treatments decreased with increasing soil depth. Different performances of AWCD values of soil microbes were observed among the same soil layers under the legacy effect of straw returning methods: in the 0 ~ 15 cm soil layer, D15 > D35 > D50 > D0; in the 15 ~ 35 cm soil layer: D35 > D50 > D15 > D0; in the 35 ~ 50 cm soil layer: D50 > D35 > D15 > D0. The legacy effects of different straw returning methods increased the McIntosh index throughout the entire soil profile (0-50 cm), with no significant effects on the Shannon index and Simpson index. The utilization capacity of soil microorganisms for carbon sources was different under the legacy effect of straw returning. Under the legacy effect of straw returning treatments, soil microbial communities exhibited varying abilities to utilize carbon sources, with the soil microbial utilization capacity for carbon sources in the 0 ~ 50 cm soil layer ranked as follows: D35 >D50 > D15 > D0. In each soil layer, the treatment with the highest carbon source utilization capacity was D15, D35, and D50 from top to bottom respectively. The most common carbon sources metabolized by the microbial community under the legacy effect of different straw returning methods were sugars, amino acids, carboxylic acids, and polymeric carbon sources. Therefore, employing a deep mixing straw returning method in heavy clay loam soil is an effective agricultural management measure to enhance microbial community functional diversity and carbon source utilization capacity.