The bi-grid modulation collimator is a significant way for imaging solar flares in hard x rays. It implements many subcollimators that consist of separated grid pairs (so-called front grid and rear grid) whose line orientations are parallel. However, when the twist of the front grid with respect to the other will be induced during testing of the bi-grid modulation collimator in the ground verification phase, the line orientation of the grid pairs are no longer parallel. Knowledge of the relative twist between the rear grid and the front grid is very helpful in improving the imaging quality of the bi-grid collimator. However, because of the wide spacing between grid pairs and the requirement of high measurement accuracy, it is a challenge to measure the relative twist. To meet this demand, a method based on the spherical wave Talbot effect is proposed. The Talbot images of the front grid and the rear grid are imaged on the same plane, respectively, through two proper spherical waves. The relative twist can be figured out through the angle between the stripes in the Talbot images of the front grid and the rear grid. In experiments, the measurement accuracy of the relative twist angle can reach 9 arcsec in the range of 370 arcsec. It demonstrates that this method can effectively measure the relative twist between the grid pairs with very high accuracy. © 2021 Optical Society of America