Focusing on the spatial geometric feasible assembly sequence planning and optimising, a novel approach entitled 3D-GCA is proposed based on the method of Geometric Constraint Analysis (GCA) incorporated with the Oriental Bounding Box (OBB) and Separation Axis Theory. Accordingly, the corresponding computer-aided 3D assembly sequence planning system is developed and two cases are studied in detail. The analysis results demonstrate that, with 3D-GCA method, the spatial assembly precedence relations (APRs) can be calculated out correctly and completely. Thereby, all the 3D geometric feasible assembly sequences can be derived automatically. Moreover, an optimisation algorithm is designed based on 3D-GCA method, by which, the optimal assembly sequences can be calculated out according to two criteria, e.g., assembly angle and assembly direction. The 3D-GCA method and its algorithms can provide significant assistance in the spatial geometric feasible assembly sequences reasoning and the optimal sequences selection.