Characteristics of the f θ lens system based on diffraction alignment technique are analyzed, thereafter a novel diffractive long trace profiler (DLTP) for large aspherical optics, especially for those optical component used in synchrotron radiation, is introduced. Details of the DLTP such as the principle of its optical subsystem, characteristics of the diffraction pattern, data processing methods are investigated. A prototype of the DLTP has been developed, its scanning range is 370 mm, and the slope resolution is better than 0.25 μrad. Experimental results show that the prototype has excellent stability and precision even in rather poor circumstance without temperature controller and air condition, and the measurement stability is about 0.7 μrad/200 s, measurement precision within whole scanning range is 1.14 μrad.