In the textbook of

Polymer Chemistry

(5th Edition) edited by Prof. Pan Zuren

for the unsealed system

the initial concentration of reactant was set as 1

when deriving the kinetic equation of reversible equilibrium linear polycondensation. Then

as the unit of initial concentration of reactant was ignored in the process of equation arrangement

inconsistence of parameters unit existed in the formula of polymerization degree

which was derived from kinetic equation. In order to solve the irrationality of the polymerization degree formula

the concentrations of reactants and products were set and retained in the equation arrangement process. Finally

by solving practical problems correctly

the accuracy the formula polymerization degree derived in this paper was proved.