This study combined the carburizing heat treatment (CHT) with the ultrasonic surface rolling process (USRP) to improve the rotary bending fatigue properties of 18CrNiMo7-6 alloy steel. Based on the fatigue limit (σ-1) obtained experimentally, the evolution of gradient compressive residual stress (CRS) of CHT and CHT + USRP treated specimens was analyzed below the fatigue limit stress level. Subsequently, the evolution mechanism of gradient CRS of the two reinforced specimens was discussed and analyzed from the opposing effects of austenite transformation and dislocation density change on the evolution of CRS. The results demonstrated that the selective bending fatigue performance of the alloy was improved obviously after the introduction of CRS, but the changed of fatigue limit is not obvious. The durability under other loads (such as cyclic tension/compression, torsion loads and etc.) needs further study. The gradient CRS evolution of CHT specimens was mainly due to large residual austenite transformation and small dislocation density change. Therefore, the volume expansion caused by residual austenite transformation gradually increased the gradient compression residual stress of CHT specimens with an increased number of cycles. In contrast, the degree residual stress evolution of CHT + USRP specimens was more affected by the dislocation density, and the gradient compression residual stress gradually decreased with an increased number of cycles. Finally, the residual stress evolution rate of the two reinforced specimens was divided into two stages: fast and slow.