A model of an open ladder-type three level noninversion lasing system is presented in this paper. A steady state linear analytical solution, and the effect of the probe and driving fields detunings varying on the noninversion lasing gain, dispersion and population difference are investigeted. It is found that the open ladder-type system can obtain the noninversion lasing gain much bigger than that obtained in the corresponding closed ladder type system; the variation rule of the gain, dispersion and population difference in the open ladder type noninversion lasing system is much different from those in the open Λ-type and simple three level noninversion lasing systems.