To grasp the fusion combustion characteristics of multiple pool fires adjacent to each other in a shaft with side slits, we carried out tests on the fusion flames formed by two, three, and four pool fires in a 2 m-high test shaft. The influence of the spacing between the pools and the width of the side slits of the vertical shaft with typical arrangements of multiple pool fires was analyzed, and the results were compared with free combustion test results of the same multiple pool fires without the constraints of the shaft structure. Results show that the unique jet air flow formed by the shaft with side slits changed the flame fusion form between multiple pool fires and the thermal feedback to the fuel surface. The fusion rotating flame formed in the shaft strengthened the combustion efficiency and raised the flame height. With fixed structure of shaft, relative strong fusion rotating flame was formed by adopting a pool placement form that enables the fuel surface of each pool to receive the heat feedback of the fusion flame uniformly, and the generating capacity of the fusion rotating flame was strengthened by increasing the number of pools. The parameters of the fusion rotating flame were affected by the coupling of the number and relative position of the fuel pools and the width of the slits on the side of the shaft. With proper slit width on the side of the shaft and the placement method that allows the fuel surface of each pool to evenly receive the heat feedback of the fusion flame, the combustion efficiency was increased by nearly 3.7 times and the peak flame height by more than 30%. In addition, on the basis of the idea of buoyant pool flame superposition circulation, the theoretical models of the burning rate and flame height of the typical multi-pool fusion flame in the shaft were derived and fitted.