In this paper, an industrial cold experiment was conducted for studying regulating characteristics of a loop seal in a 65 t/h oil shale-fired circulating fluidized bed (CFB) boiler. The starting characteristic of the loop seal, and the effect of the supplying air and the fluidizing air in the loop seal were investigated. Compared with other regulating modes, the combined regulating mode of keeping the fluidizing air rate constant and regulating the supplying air rate can make the loop seal obtain better regulating quality, and offer more reliable guarantee for steady operation of the CFB boiler as well. In order to prevent circulating material depositing and slagging at the bottom of the loop seal, it was suggested that the fluidizing air rate and the supplying air rate is 2-3 and 1.2-1.5 times of the minimum fluidizing velocity of circulating material, respectively. These experimental results may be used as a reference for regulating the loop seal of the 65 t/h CFB boiler in hot condition and designing a new loop seal.