Spout deflection is a common instability phenomenon, which has been widely encountered in spouted and spout-fluidized beds. However, the spout deflection has not been quantified experimentally. In the present work, the alternating spout deflection behavior is captured by employing the particle image velocimetry (PIV) method and quantified based on PIV vector fields over a wide range of variables in terms of bed heights and spouting velocities. The results show that the static bed height has an obvious effect on the alternating spout deflection. The amplitude of the alternating spout deflection keeps stable at first and then reduces quickly with increasing static bed height. The drop tendency is ascribed to the appearance of spout incoherence behavior in which the particle blockage periodically forms and explodes in the spout region. This explosion contributes to uniform particle spurting and reduces the height difference between two annulus regions and the spout deflection amplitude. The amplitude of the alternating spout deflection is stable with increasing spouting velocity, which can be owed to the self-locking phenomenon. Specifically, the particles in the annulus region on one side cannot be transported away due to the resistance of the deflected spout and thereby preventing the further deflection of the spout. Spouting velocity has a neglectable influence on the regularity and main frequency of the alternating spout deflection. This work unveils the hydrodynamics of spout deflection and provides a methodology for experimental investigation of spout deflection in spouted beds.