Isoplanatic angle is one of important atmospheric turbulence parameters which has play a significant role in system design and compensation effect of adaptive optics. A apodized mask with triple concentric annuluses is introduced and applied to high precision measurement of isoplanatic angles. Principle and measurement technique are presented, and isoplanatic angle is measured at night in Xi′an. The conclusion of theoretical derivation shows that the apodized mask can modulate irradiance of star light received as a “5/3” power of height and improve measurement precision. The measurement results shows that isoplanatic angles in a range 5~10 μrad at night in Xi′an agree with that in the same latitude regions. The suggestions for improving experimental device are proposed.