This paper deals especially with spouting stability in a slot-rectangular (prismatic) spouted bed. The flow stability was characterized by the pressure drop evaluation (the uniformity and amplitude of the fluctuations) and visual observations (the flow symmetry). The effect of several geometrical conditions, such as the inlet design, prismatic angle and draft plates on the bed behaviour was investigated for different particles and bed inventories exposed to different air flow rates. The prismatic angle was found to have a strong impact on the spouting characteristics. A method to improve significantly the spouting quality and to make the spouting stability independent on the gas inflow velocity is presented. Compared to the reference geometry the prismatic angle was changed to a higher value and the apparatus was equipped with draft plates. Whereas, to achieve a stable spouting in a wide range of the air flow rates the design of draft plates should be customized to the bed inventory. Dependent on the static bed height full or open-sided draft plates should be used. In the proposed apparatus implementation, the maximum spouting velocity was almost eliminated for large particles (Geldart D), i.e. a highly coherent dense spouting can pass continuously into the dilute-like stable regime, without stability loss at intermediate gas velocities. The spouting of Geldart B particles is also improved.