Aiming at the difficulties and problems of online measurement of pulp kappa number during batch pulping, this paper proposes a new measuring idea, which is to predict the lignin content ill Pulp through the measurement of dissolved lignin in cooking liquor with spectral technology, The basic and modified mathematical model of kappa number during sulfite pulping as well as the end point controlling model are put forward in detail. The site running result shows that the introduction of modified factor epsilon, which represents the change of the total lignin amount added into the digester, is effective. Therefore, the online measurement of kappa number and the determination of the end point of pulping can be achieved.