As there are issues such as high cost and low efficiency in reducing the flank distortions with threaded grinding wheel, an approach to improve the issues in design process is explored in this paper. Firstly, according to the contact trace analysis on worm wheel grinding, a novel method to calculate the magnitude of gear flank twist is proposed and the influence of crowning and helix angle on the magnitude of flank twist is analyzed. Secondly, an optimization method of crowning modification curve is proposed, which divides crowning curve into three parts to calculate the lengthwise crowning of the left and the right parts respectively. Then within allowance of gear design requirement, the flank distortions are lessened by adjusting scale factors and flank crowning. At last, an example verifies that the method could eliminate the gear twist and reduce the corrected errors.