It is still an unresolved issue to test the assembly structures inside products in industry applications. Spatial sampling criteria based on the single view imaging system are proposed for recognition of structural objects. There must be a maximum rotary step for an object within which the least structural size to be tested is ascertained. Rotating the object by the step and imaging it and so on until a 360°turn is completed, an image sequence is obtained that includes the full structural information for recognition. The maximum rotary step is restricted by the least structural size and the resolution of the imaging system. Based on the spatial sampling criteria for structural recognition, the theory of fast recognition of structural objects is proposed. It is verified that objects could be fast recognized at a single or some limited orientations with a single camera by analyzing the smooth correlations among the image sequence sampled from 0° to 360° orientation. The theory is applied to the X-ray digital radiography system designed for online automatic inspection of interior assembly structures of complex products. Experiments show that the average identification takes less than 5 s in test progress at the wrong recognition rate of 4.5%.