Piston aberration in isoplanatic error does not affect imaging quality and requires no calibration; therefore, effective anisoplanatic error should eliminate piston aberration. The approach to compute the piston-removed isoplanatic angle is proposed in this study, which involves measuring double star wavefront error with Shack-Hartmann wavefront sensor. First, the value of the anisoplanatic error in various atmospheric environments is computed using Sasiela and Van Dam analytical expression for angle anisoplanatic error. Thereafter, the process of wavefront error measurement in an actual adaptive optics system with different atmospheric conditions is simulated and the anisoplanatic error is computed using phase screen method. The numerical simulation findings and theoretical calculations are similar. Meanwhile, the correspondence between wavefront error and piston-removed isoplanatic angle is computed. Finally, the anisoplanatic error of double stars is estimated and the piston-removed value of the isoplanatic angle is computed in Lijing 1.8-m astronomical telescope. The finding is confirmed using the approaches of differential image motion and stellar scintillation. The experimental findings exhibit that the piston-removed isoplanatic angle differs slowly in the temporal dimension and rapidly in the spatial dimension, and the importance of the piston-removed isoplanatic angle will be lost with long distance. Based on the approach, the correspondence between wavefront error and piston-removed isoplanatic angle in other atmospheric models and telescopes could be computed and it offers a basis for the location of the beacon.