Considering the speed control characteristics of a low-speed two-stroke marine diesel engine, a modified active disturbance rejection control（ADRC）strategy was proposed, which is more suitable for low-speed diesel engine speed control. In order to reduce the impact of inherent speed fluctuations in diesel engines under small error condition, and to reduce the recovery time under big error situation, a strategy combined with linear extended state observer（LESO）for estimating and compensating the total disturbance was designed for constructing an improved ADRC method, in which linear feedback control law（EFCL）was employed when the absolute value of the error was smaller than 1, while nonlinear EFCL with “large error, large gain”was used in the else condition. To verify the proposed method, a cylinder-by-cylinder mean value engine model（MVEM）that can embody the cyclic speed fluctuations was built for fully considering the inherent speed fluctuations of diesel engines. Based on the proposed engine model, the performance of the proposed controller was verified through a hardware-in-loop（HIL）testing platform. The results demonstrate that the improved ADRC can balance multiple static and dynamic performance indexes. The proposed controller not only has good robustness to changes in the rotational inertia of the diesel engine, but also is insensitive to variations in its own parameters. Moreover, it is easy to tune the control parameters, which is of great significance for the practical application of ADRC technique in low-speed two-stroke marine engines.