The invention relates to a lightweight BIM big data online visualization method and system. The method includes the following steps that lightweight analysis based on semantic guidance is conducted on a BIM scenario component set to obtain a lightweight component base; the interior structure of a BIM building is automatically partitioned to generate multiple relatively independent spaces and a relation structure chart of the relatively independent spaces; a multilayer data index structure with multi-granularity decomposition is established according to the relation structure chart and the component set in each space; the lightweight component base is called according to the multilayer data index structure to achieve progressive visualization of the BIM building, and in the visualization process, with each space as the unit, data in the next space relative to the current space is cached and data in the last space is deleted. Compared with the prior art, the method and system have the advantages of automation, lightweight, fine granularity and the like.