Besides the state equation there exists another cubic algebraic equation about μf in the form of μ_f~3 + pμ_f + q = 0, which relates parameters, temperature T, chemical potential μ_f, and net quark number n_(ff) (flavor f) for a baryon-rich quark-gluon plasma (QGP). A criterion may be acquired simply according to Cardan formula of the solution of the above equation, which gives naturally a condition: if n << 2πT~3/3, one may approximately use the conservation of specific entropy, and then the set of hydrodynamic equation of baryon-rich QGP may be reduced to the set of hydrodynamic equation for baryon-free QGP. There is another cubic algebraic equation about μf in the form of μ_f ~ 3 + pμ_f + q = 0, which parameters, temperature T, chemical potential μ_f, and net quark number n_ (ff) (flavor f) for a baryon-rich quark-gluon plasma (QGP). A criterion may be acquired simply according to Cardan formula of the solution of the above equation, which gives naturally a condition: if n << 2πT ~ 3/3, one may be use the conservation of specific entropy, and then the set of hydrodynamic equation of baryon-rich QGP may be reduced to the set of hydrodynamic equation for baryon-free QGP.