Conservation tillage has been widely adopted to improve soil organic carbon (SOC) accumulation. To assess the effects of different tillage and residue-returning modes on SOC mineralizability and accumulation, a field experiment was conducted in 2015, including two tillage modes, i.e., no-tillage (NT) and rotary-tillage (RT), as well as three straw-returning modes, i.e., no straw returning (N0), wheat straw returning (WR), and wheat–maize straw retuning (WM). Wheat–maize root, maize straw, and wheat straw were added to soil samples during laboratory incubation. The results showed that NT significantly increased SOC concentration by 26.75% at a depth of 0–10 cm (p < 0.01). Although NT significantly decreased SOC mineralizability at soil depths of 0–10 cm and 10–20 cm (p < 0.05), the residue did not significantly affect SOC mineralizability in the 0–20 cm layer. The potentially mineralized C (Cp) and organic labile C fraction (C1) increased with the increase of residue amount. A significant correlation (R2 = 0.662) was observed between C1 and SOC concentration. Notably, SOC concentration and mineralizability without straw returning were insignificant between N0 and WR in the 0–10 cm and 0–20 cm layers, respectively. This suggests that NT with WR is a promising strategy to increase SOC concentration and decrease mineralizability.