This paper deals with the design of a compact gearbox for the DTU 10 MW reference offshore wind turbine. An innovative gearbox concept consisting of a fixed planetary stage and a differential compound epicyclic stage is proposed. Power splitting and compound epicyclic transmission technologies are employed, which can effectively reduce the gearbox size. The power transmission principle of the gearbox is described and power distribution ratios in two transfer paths are calculated via the geometrical and mechanical relationships among the gearbox components. The gearbox is designed based on the design loads and fatigue criteria by referring to relevant international standards. A high-fidelity drivetrain numerical model is established by means of multi-body system (MBS) approach. Then, the power distribution ratios in two transfer paths are compared between the simulation results and the design values with a good agreement. Resonance analysis of the drivetrain model is conducted by means of the Campbell diagram, energy distribution of components, and the results show that no severe resonance phenomena appear in this drivetrain model. Additionally, the load-sharing behaviour of the gearbox model is assessed under different environmental conditions, and the results indicate that the compact gearbox has a good load-sharing performance. Such compact gearbox design, which fulfils all the design requirements for offshore applications, could be a good alternative for offshore wind turbines.