The determination of numerical reconstruction distance is key to recover the wavefront at focal plane in digital holography.In this paper,we propose a new autofocus method based on angular spectrum method(ASM).The proposed method takes successive Fourier transform after 1st order spectrum selection,and then calculates the summation.It saves operations compared to classic autofocus functions.When an exhaustive z-axis search is performed,the proposed method obtains unimodality coincided with the results from four classic autofocus functions.Moreover,the proposed method is more time-effective,which is the optimal one for ASM.