In order to test large and off-axis aspheric surfaces without the aid of other null optics,a new method subaperture stitching interferometry(SSI) is introduced.The basic principle of the technique is analysed,and the synthetical optimization stitching model and effective stitching algorithm are established based on homogeneous coordinates transformation and simultaneous least-squares method.The stitching software and prototype for testing of large aspheres by SSI are devised and developed.An off-axis asphere with the aperture of 376×188 mm2 is tested by this method.For the comparison and validation,the asphere is also tested by null compensation,the synthesized surface map is consistent to the entire surface map from the null test,and the difference of PV and RMS error between them is 0.047λ and 0.006λ,respectively.So it provides another quantitive measurement for testing large aspheric surfaces and off-axis aspheres besides null-compensation.