In the broad sense, unbalanced-drilling is not only an effective technology of exploiting low pressure and low permeability formation and exhausted formation, but also a drilling and completion technique for finding and exploiting formations which are different pressure systems. Unbalanced-drilling pressure difference can be obtained by use of single-phase fluid, aerated fluid, tiny glass bead, and tiny plastic bead. If unbalanced-drilling pressure difference is obtained by different ways, drilling cost will be much variable. Under allowable formation pressure, single-phase and low-density fluid was injected from surface. When it was circulated in the annular space, it can stimulate formation fluid into annular space so that we can obtain the designed unbalanced-drilling pressure difference, working-on process, and theory of annular pressure system. How to select single-phase fluid injected from surface, and to obtain of initial unbalanced-drilling pressure difference, calculation and control technique of characteristic factor of annular multiphase fluid dynamics, control and calculation of injection quantity of formation fluid, control technique of unbalanced-drilling casing pressure and standpipe pressure, were discussed and analyzed in this study in detail. This technology has been applied to Northern China Oil Field successfully.