In the frequency control of interconnected power systems， the fixed coefficient method is generally employed to set the frequency bias coefficient B . In this methodological framework， the automatic generation control（AGC） regulating quantity cannot accurately track the system power bias， and as a result the frequency overshoot/undershoot and even frequency stability could be severe issues. Given this background and considering the fact that different setting values of the primary frequency control dead zones are employed in thermal units and hydro units， a three-stage B -coefficient method is presented， with an objective of obtaining a better system regulation performance and more accurately tracking system power bias. China Southern Power Grid is employed to demonstrate the developed method by simulation studies and on-site testing， and it is shown by both simulation and on-site results that the three-stage B -coefficient method could significantly reduce the frequency overshoot/undershoot problem， improve the frequency quality as well as the frequency control performance of interconnected power systems.