The invention relates to the field of railway, and discloses a simply supported beam deflection test device and method. The test device comprises first and second displacement sensors which are used for testing first and second longitudinal bridge direction displacement of a position at which a cross section sensor of a simply supported beam is positioned; and a data processing device which is used for obtaining the turning angle of the cross section of the simply supported beam according to the first and second longitudinal bridge direction displacement and vertical height between the first and second displacement sensors so that deflection of the simply supported beam is acquired. The test device or method has no limitation of topographic and hydrological conditions in testing, and influence of resonance and wind load on the test system can be eliminated.