Abstract: Circulation ratio is very important to the running condition monitoring and key parameters controlling of circulating fluidized bed (CFB) boilers, however, the value is generally very difficult to be measured directly. To solve this problem, a soft-sensing method was proposed for the circulation ratio by establishing and simplifying balance equations through mechanism analysis of chemical process in the furnace and material recirculating process in the CFB boiler according to the law of mass conservation, based on which key intermediate variables were calculated, such as the fly ash content, separator efficiency and so on, and subsequently the circulation ratio was obtained by simulation. Results show that a linear relationship exists between the circulation ratio and the density of particles in dilute region per unit load; the calculated circulation ratio is quite approximate to the design value of CFB boiler, proving the soft-sensing method to be effective and available.