To address the design and operational optimization issues of falling film heat exchangers，high‑definition photography and digital image processing techniques were proposed to investigate the wettability of falling film flow outside multi‑row horizontal tubes.By constructing a ten‑row horizontal tube visualization experimental platform，liquid film distribution under three tube diameters，four tube spacings，and four inter‑tube flow patterns was analyzed.The “wetting length” indicator was proposed，and its variation patterns with tube diameter （d），tube spacing （s），and liquid film Reynolds number （Re_l） were analyzed.The results are as follows：increasing flow rate，tube diameter，and tube spacing all promote wetting length growth；when s/d=1.25，the optimal tube diameters for droplet，columnar，and sheet flow patterns are 16，25，25 mm，respectively；when Re_l≤505，the wettability ranking is l_{16 mm}>l_{19 mm}>l_{25 mm}，while when Re_l>505，it is l_{25 mm}>l_{16 mm}>l_{19 mm}.Correlation formulas for wetting ratio and total wetting length were fitted and compared with experimental values and literature experimental values，with average errors less than 10% and 8%，respectively，showing good agreement.The conclusion is that this can provide theoretical support for heat and mass transfer experiments and heat exchanger design.