Perpendicularity error is a critical geometric error in multi-axis motion platforms, indicating the vertical deviation of machine tool components during movement. This deviation significantly affects spatial positioning errors, particularly as the stroke length of the moving axis increases. Various methods exist for measuring perpendicularity error, including physical gauges, volumetric measurements, CCD measurements, planar gratings, laser trackers, and laser interferometry. However, these methods often suffer from drawbacks such as operational inconvenience, high costs, or inability to perform online measurements. This paper presents a multi-axis laser interferometric device for measuring perpendicularity, utilizing components like laser emitters, pentagonal beam splitters, prisms, straightness interferometers, and reflectors. With a well-designed optical path, this device can be integrated into the multi-axis motion platform, enabling efficient and precise online measurement of perpendicularity. A comparison test against a granite protractor showed a deviation of 1.4″ and a normalized deviation E_n<1, confirming the method's feasibility and accuracy.