The invention provides a modularly assembled foldable backrest tube. The modularly assembled foldable backrest tube comprises a measurement component and an extension component, wherein one end of theextension component is in hinge joint with one end of the measurement component, and the measurement component can be turned over around one end of the extension component. The modularly assembled foldable backrest tube can be connected with a plurality of extension components in series so as to realize combinations of different length, wherein each extension component can be turned over relativeto neighboring extension components. The modularly assembled foldable backrest tube has the beneficial effect of being capable of being sent into an air bellow after being straightened section by section in a folded state, thus, repeated disassembling and assembling of the backrest tube is avoided, moreover, the whole body of the backrest tube is provided with a length graduation scale which is convenient for measurement and location, and an aim of reducing operation intensity and improving working efficiency can be realized. Since the sections can be quickly folded together with each other,the backrest tube is convenient and efficient to operate and easy to transport and take, and can be used for repeated measurement for multiple measurement points under various working conditions for alarge-scale air bellow for which outside operation space is limited.