Abstract: For the purpose of resolving the problem of performance deterioration introduced by inaccurate phase compensation with the existing coherent averaging line spectrum detector, a modified coherent detector is proposed. The three point interpolation in frequency domain is applied to obtain accurate phase difference estimate between segments when the segmented length is not the integer times of the signal period. Then the segmented data are multiplied a complex coefficient to remove the phase difference and synchronize the phases of all segments before the coherent averaging. Theoretical analysis shows that there will be a gain of 3.9 dB at most by using the modified detector. The detection performances of the incoherent averaging power spectrum detector (AVGPR), the phase coherent averaging detector, the modified detector are compared with each other by computer simulations, and the results coincide basically with the theoretical analysis, which show the superiority of the modified detector to the former two detectors.