The invention relates to the technical field of aircraft engine assembling, and particularly provides an aircraft engine fan casing and an assembling method thereof. The fan casing is divided into twooppositely-opened casing bodies. An inner ring of each oppositely-opened casing body is of an integral ring structure and is cut into four ring bodies, and each ring body is provided with a single-side retaining groove structure. In the assembling process, check blocks are fixed through clamping grooves with lower margin plates of stator blades installed therein, firstly, one stator blade provided with the check block and the other stator blades are assembled onto each oppositely-opened casing body and fixed to the casing in the peripheral direction, then the inner ring slides in along the lower margin plate of the stator blade, after the retaining groove of the inner ring makes contact with the check block, movement of the inner ring in the sliding-in direction is limited, and the othercasing body is also provided with one stator blade provided with the check block. By the adoption of the method, the movement of the inner rings in the sliding-in direction is limited, the two oppositely-opened casing bodies are assembled together, and then, the stator blades and the inner rings are positioned.