  We calculate the longitudinal flow decorrelation coefficients, i.e.,
$r_n(\eta,\eta_r)$ for $n=2,3$, in the presence of hydro-like flow and the
global momentum conservation (GMC) constraint. The longitudinal flow
decorrelation is weakened due to the GMC constraint. The GMC effect is
sensitive to the total number of particles involved in GMC, the average
longitudinal momentum, the transverse momentum, and the reference
pseudorapidity. Our results of the $r_2(\eta,\eta_{rA})/r_2(\eta,\eta_{rB})$
ratio between two reference pseudorapidity bins are consistent with the
experimental measurements. We predict that the modification effect of GMC on
longitudinal flow decorrelation is more noticeable at BNL Relativistic Heavy
Ion Collider energies than at CERN Large Hadron Collider energies. Our finding
provides a new perspective for understanding the longitudinal flow
decorrelation in relativistic heavy-ion collisions.
