The invention concerns a circulating fluidized bed reactor based on a multi-inlet cyclone, comprising an elongated riser channel (5; 25), which has a lower portion (4; 24) and an upper portion, whereby solid matter and fluidization gas can be fed into the riser through the lower portion (4; 24). In the upper portion of the riser channel there is a a multi-inlet cyclone which comprises a separation chamber (7; 27), with a set of guide vanes (6; 26), an annular space (8; 28) connected to the separation chamber, an outlet (11; 31) and a return channel (10; 30). According to the invention, the set of guide vanes (6; 26) of the multi-inlet cyclone, the cyclone chamber (7; 27) and the annular space (8; 28) are annularly fitted about the riser channel (5; 25). Pursuant to the invention a solution having advantageous flow dynamics is obtained. The reactor can be used e.g. as a dryer or a steam boiler.