The region localization methods of the positive first-order spectrum rely on the prior template or the tilt angle of the reference wave， which can be problematic. Therefore， a new method for locating the positive first-order spectrum that uses the maximum spectral phase value is proposed. The theoretical distribution of the spectral phase in pre-magnified digital holography is analyzed， which reveals that the maximum phase point is located in the center of the positive first-order term spectrum. Furthermore， the minimum distance between the centroids of the three regions （positive first-order spectrum， negative first-order spectrum， and zero-order term spectrum） and the maximum phase point is used to judge and locate the positive first-order spectrum region. Finally， using a micro-hole array and phase step as examples， experiments are carried out， the results of which are consistent with the theoretical analysis. Specifically， they demonstrate that the absolute error is 3.7 μm， and the standard deviation of depth is 4.2% when measuring the phase step. Therefore， the proposed method can accurately locate the positive first-order spectrum region. Moreover， it does not depend on the prior template or the tilt angle of the reference wave， which provides an automatic spectrum localization method for spatial filtering in off-axis digital holographic microscopy.